502 lines
18 KiB
Text
502 lines
18 KiB
Text
== Lesson 19: Text That Looks Good
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image::learn/19-text.png[The finished lesson, 480]
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Every word you have put on screen since lesson one came out of `overlayPrint`,
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and `overlayPrint` has exactly one font: a fixed-width block of pixels built
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into the engine, the same size forever, in whatever colour it happens to be.
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It is perfect for a frame counter. It is wrong for a title, a score, a menu, or
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anything a player is meant to look at rather than read past.
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This lesson loads a real font off disk, prints with it, colours it, measures it
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so you can centre things, and shows you what printing text actually costs. At
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the end you will have a title screen that looks deliberate instead of
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accidental.
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=== Set the Overlay to a Size You Can Think In
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Start a new file called `text.singe` with two lines:
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[source,lua]
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----
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dofile("Singe/Framework.singe")
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overlaySetResolution(discGetWidth(), discGetHeight())
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----
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The first line you have written many times; it loads the framework, which
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defines `DIR`, the `SWITCH_*` names, and the `FONT_QUALITY_*` names you are
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about to use.
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The second matters more than it looks. The overlay -- the surface everything
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you draw lands on -- starts at *half* the size of the world. A game with no
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video has a 720 by 480 canvas, so the overlay starts at 360 by 240, and a
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36 point font on a 360 pixel wide overlay is enormous. `overlaySetResolution`
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replaces the overlay with one of the size you ask for, and from then on every
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coordinate you give, and everything `overlayGetWidth` and `overlayGetHeight`
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tell you, is in that space.
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`discGetWidth()` and `discGetHeight()` answer the size of the video when there
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is one and the size of the canvas when there is not, so that one line gives you
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an overlay that matches the world whichever kind of game this is. Call it once,
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at the start, before you draw anything.
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=== Load a Font
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Under that, add:
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[source,lua]
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----
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fontQuality(FONT_QUALITY_BLENDED)
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local titleFont = fontLoad("Singe/FreeSansBold.ttf", 48)
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----
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`fontLoad` opens a TrueType font file and hands you back a number -- a *handle*
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-- that you use whenever you want that font. The second argument is the point
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size, and it is fixed for the life of the handle. There is no way to change the
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size of a loaded font afterwards. If you want the same face at three sizes, you
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call `fontLoad` three times and keep three handles.
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`Singe/FreeSansBold.ttf` is the font the engine unpacked into the `Singe`
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folder in your work folder the first time you ran anything. Every copy of Singe
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has it, so you can rely on it. Any other `.ttf` file works the same way; put it
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beside your script and load it as `DIR .. "myfont.ttf"`.
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Now print with it:
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[source,lua]
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----
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function onOverlayUpdate()
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overlayClear()
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fontPrint(24, 60, "ASTEROID PATROL")
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return OVERLAY_UPDATED
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end
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----
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Run it. Big white letters near the top left.
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Notice what you did *not* do: you never told `fontPrint` which font to use.
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`fontLoad` makes the font it just loaded the selected one, and `fontPrint`
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always draws with whatever is selected. With more than one font loaded you
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choose between them with `fontSelect`, which you will do in a moment.
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Notice also that `24` and `60` are pixels, not character cells. This is the
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difference between `fontPrint` and `overlayPrint` that trips people up:
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`overlayPrint(2, 2, ...)` means two letters in and two lines down, while
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`fontPrint(2, 2, ...)` means two pixels in and two pixels down, which is
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jammed into the corner. The `y` you give is the *top* of the text, not the
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baseline it sits on.
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=== Colour
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`fontPrint` takes three arguments and none of them is a colour. The colour
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comes from somewhere else:
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[source,lua]
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----
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colorForeground(255, 211, 90)
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fontPrint(24, 60, "ASTEROID PATROL")
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----
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`colorForeground` sets red, green, and blue from `0` to `255`, and an optional
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fourth number for opacity. It is not an argument to anything; it is a setting
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that the engine remembers. Everything drawn after it comes out in that colour
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until you change it again, exactly the way `overlayBox` and `overlayLine`
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already work. So the pattern is always the same: set the colour, then draw.
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This is the first thing people get wrong with fonts, and the second thing is
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coming up in a few pages.
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=== Quality
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`fontQuality` decides how the letters are rasterized -- how the curves of a
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letter are turned into pixels -- and there are three choices. Try each one by
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putting it above the `fontLoad` line and looking at the result closely.
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`FONT_QUALITY_SOLID` is the fastest. Every pixel is either the letter or it is
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not, so the diagonals and curves come out with visible stair steps. At 48 point
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it looks cheap. At 12 point on a small overlay it can look *sharper* than the
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alternatives, which is sometimes what you want.
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`FONT_QUALITY_SHADED` smooths the edges, but it does it by blending them into
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the current background colour, and it draws that background as a solid
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rectangle behind the whole string. You get a label with a box around it whether
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you wanted one or not. It is the right answer for a debug readout over a busy
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picture and the wrong answer for almost everything else.
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`FONT_QUALITY_BLENDED` smooths the edges and keeps them see-through, so the
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letters sit on whatever is behind them. It costs the most and it is what you
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want nearly all the time. Use it unless you have a reason not to.
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The setting is global, not per font, and it applies from the moment you call it
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onwards. Sprites you have already made keep the quality they were made with.
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The manual's entry for `fontQuality` lists the three names and their values.
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=== Measuring, so You Can Centre Things
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Sooner or later -- usually about ten minutes in -- you want the title in the
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middle of the screen. You know the screen is `overlayGetWidth()` wide. You do
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not know how wide the text is, and there is no `fontMeasure` call to ask.
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What there is instead is `fontToSprite`. It renders a string with the selected
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font, the current quality, and the current colour, and instead of putting it on
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screen it hands you back a *sprite*: the same kind of handle `spriteLoad` gives
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you, which means every `sprite*` call you learned in lesson nine works on it.
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Including the two that measure it.
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[source,lua]
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----
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fontSelect(titleFont)
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colorForeground(255, 211, 90)
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local titleImage = fontToSprite("ASTEROID PATROL")
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----
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And then, in `onOverlayUpdate`:
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[source,lua]
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----
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local x = (overlayGetWidth() - spriteGetWidth(titleImage)) / 2
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spriteDraw(titleImage, x, 60)
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----
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That is centring, and it is the whole trick. Take the width of the space, take
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away the width of the thing, and half of what is left is the gap at each side.
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Right alignment is the same idea with less arithmetic. To put text against the
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right hand margin:
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[source,lua]
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----
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spriteDraw(scoreImage, overlayGetWidth() - MARGIN - spriteGetWidth(scoreImage), y)
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----
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`spriteGetHeight` measures the other direction, for stacking lines or centring
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vertically.
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There is a catch in that first code block, and it is the second thing everybody
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gets wrong. *The colour is baked in when the sprite is made, not when it is
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drawn.* `colorForeground` above `fontToSprite` changes the text. Putting it
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above `spriteDraw` instead does nothing at all, because a sprite is drawn with
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its own pixels and is never tinted. If your carefully coloured title comes out
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white, this is why.
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=== What Printing Costs
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The first time `fontPrint` sees a string, it does real work: it rasterizes
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every letter from the font's outlines and builds an image of the whole string.
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Then it keeps that image. The next time you print the same string, with the
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same font, quality, and colours, it skips all of that and copies the image it
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already has, which is about what drawing a sprite costs. A string you stop
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printing is let go a second later, so nothing piles up.
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That means the obvious way to write a title screen is also the fast one.
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Printing `"ASTEROID PATROL"` sixty times a second renders it once. A score that
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changes twice a second renders twice a second, on its own, with no bookkeeping
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from you.
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The one case that costs is text that is *different* every frame -- a timer in
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milliseconds, a frame counter -- because every frame is a string `fontPrint`
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has never seen. On a desktop it does not matter. On a Raspberry Pi or a
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handheld, a screenful of it will. The cure is to change the text less often:
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a timer in whole seconds is a new string once a second, not sixty times.
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So why did this lesson use `fontToSprite` at all? Because a sprite can do
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things printed text cannot:
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* *It can be measured*, which is the only way to centre text or line it up
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against the right hand side.
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* *It can be scaled and turned* with the `sprite*` calls from lesson nine.
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* *It can outlive its font*, which matters in a moment.
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Use `fontPrint` for text, and `fontToSprite` when you need one of those.
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The score in this lesson needs one of them: it sits against the right hand
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margin, so it has to be measured, so it has to be a sprite. And a sprite of a
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score is only right until the score changes. When it does, render a new sprite
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and unload the old one -- forgetting that leaks a sprite per change:
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[source,lua]
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----
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local function renderScore()
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if scoreImage ~= nil then
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spriteUnload(scoreImage)
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end
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fontSelect(scoreFont)
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colorForeground(255, 255, 255)
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scoreImage = fontToSprite(string.format("%06d", score))
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scoreShown = score
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end
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----
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`scoreShown` remembers which score that sprite says, so the drawing code can
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tell whether it is still right:
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[source,lua]
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----
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if score ~= scoreShown then
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renderScore()
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end
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----
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`string.format("%06d", score)` is plain Lua, not Singe. It turns a number into
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a string, and `%06d` means "as a whole number, at least six digits, padded with
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zeros", which is why arcade scores have leading zeros. Change it to `%d` and
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the zeros go away.
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=== Putting Fonts Away
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`fontUnload` closes a font and makes its handle invalid. Sprites you made with
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`fontToSprite` are not affected -- once rendered, a sprite has nothing more to
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do with the font -- so you can unload a font and go on drawing text made from
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it.
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One sharp edge: if you unload the font that was selected, *no* font is selected
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afterwards, and the next `fontPrint` ends your game with an error. Select
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another one first if you are going to keep printing.
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The tidy place for all this is `onShutdown`, a callback the engine calls once
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when the game is closing:
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[source,lua]
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----
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function onShutdown()
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spriteUnload(titleImage)
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fontUnload(titleFont)
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end
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----
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Singe cleans up after you when the game ends, so nothing catches fire if you
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skip this. Write it anyway. A game that releases what it loaded is a game you
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can load twice.
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=== The Whole Script
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[source,lua]
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----
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dofile("Singe/Framework.singe")
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overlaySetResolution(discGetWidth(), discGetHeight())
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local MARGIN = 24
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local TITLE_Y = 60
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local SCORE_Y = 170
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local LABEL_Y = 186
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local HINT_Y = 300
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local CLOCK_Y = 430
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local score = 0
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local titleFont = nil
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local scoreFont = nil
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local smallFont = nil
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local titleImage = nil
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local hintImage = nil
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local scoreImage = nil
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local scoreShown = nil
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local function centred(image)
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return (overlayGetWidth() - spriteGetWidth(image)) / 2
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end
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local function renderScore()
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if scoreImage ~= nil then
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spriteUnload(scoreImage)
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end
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fontSelect(scoreFont)
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colorForeground(255, 255, 255)
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scoreImage = fontToSprite(string.format("%06d", score))
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scoreShown = score
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end
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fontQuality(FONT_QUALITY_BLENDED)
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titleFont = fontLoad("Singe/FreeSansBold.ttf", 48)
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scoreFont = fontLoad("Singe/FreeSansBold.ttf", 36)
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smallFont = fontLoad("Singe/FreeSansBold.ttf", 18)
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fontSelect(titleFont)
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colorForeground(255, 211, 90)
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titleImage = fontToSprite("ASTEROID PATROL")
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fontSelect(smallFont)
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colorForeground(140, 200, 255)
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hintImage = fontToSprite("Press the fire button to score")
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renderScore()
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function onInputPressed(what)
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if what == SWITCH_BUTTON1 then
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score = score + 125
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end
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end
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function onOverlayUpdate()
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overlayClear()
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spriteDraw(titleImage, centred(titleImage), TITLE_Y)
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if score ~= scoreShown then
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renderScore()
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end
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spriteDraw(scoreImage, overlayGetWidth() - MARGIN - spriteGetWidth(scoreImage), SCORE_Y)
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fontSelect(smallFont)
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colorForeground(160, 160, 160)
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fontPrint(MARGIN, LABEL_Y, "SCORE")
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fontPrint(MARGIN, CLOCK_Y, "Running for " .. (singeGetTicks() // 1000) .. " seconds")
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if (singeGetTicks() // 500) % 2 == 0 then
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spriteDraw(hintImage, centred(hintImage), HINT_Y)
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end
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return OVERLAY_UPDATED
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end
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function onShutdown()
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spriteUnload(titleImage)
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spriteUnload(hintImage)
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if scoreImage ~= nil then
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spriteUnload(scoreImage)
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end
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fontUnload(titleFont)
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fontUnload(scoreFont)
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fontUnload(smallFont)
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end
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----
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Run it and hold the fire button. The title and the hint sit centred, the score
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climbs against the right margin and is re-rendered only when it changes, the
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hint blinks, and the label and the clock are printed with `fontPrint` every
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frame -- the clock in whole seconds, so it is a new string only once a second.
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=== What Just Happened
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[source,lua]
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----
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titleFont = fontLoad("Singe/FreeSansBold.ttf", 48)
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scoreFont = fontLoad("Singe/FreeSansBold.ttf", 36)
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smallFont = fontLoad("Singe/FreeSansBold.ttf", 18)
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----
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One file, three sizes, three handles. Each one costs memory for its own set of
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rendered glyphs, so load the sizes you use and no more. Three is normal. A
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dozen is a smell.
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After these three lines the selected font is `smallFont`, because loading
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selects. That is why the code calls `fontSelect(titleFont)` before rendering
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the title: the last thing loaded is not the thing you want first.
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[source,lua]
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----
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local function centred(image)
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return (overlayGetWidth() - spriteGetWidth(image)) / 2
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end
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----
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Written once as a function because it is used twice, and because the next time
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you want something centred you will want it again. Give the arithmetic a name
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and you never have to read it again.
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[source,lua]
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----
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if (singeGetTicks() // 500) % 2 == 0 then
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spriteDraw(hintImage, centred(hintImage), HINT_Y)
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end
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----
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`singeGetTicks()` is the number of milliseconds since the engine started. `//`
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is division that throws away the remainder, so `singeGetTicks() // 500` counts
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half-seconds, and `% 2` is the remainder after dividing by two, which is `0`,
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`1`, `0`, `1`... So the hint is drawn for half a second and skipped for half a
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second. Drawing nothing is how you make something blink.
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[source,lua]
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----
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fontSelect(smallFont)
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colorForeground(160, 160, 160)
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fontPrint(MARGIN, LABEL_Y, "SCORE")
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----
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Both settings are needed here, and it is worth being clear about why. Drawing
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a sprite does not change the selected font or the colour, so the two
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`spriteDraw` calls above leave both exactly as they found them. But the setup
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code selected three different fonts in turn, and `renderScore` selects another
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one whenever the score moves, so the selection when this line runs is whichever
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one happened to be chosen last. Do not try to keep track of it. Select the font
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and set the colour immediately before you print, every time. It is two lines
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and it removes a whole category of puzzled evening.
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=== Try It
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. *Change the quality.* Put `FONT_QUALITY_SOLID` in the `fontQuality` line and
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look at the curve of the `S` in `ASTEROID`. Then try `FONT_QUALITY_SHADED`
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and watch what happens behind every string.
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. *Make the hint yellow.* Move the `colorForeground(140, 200, 255)` line from
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above `fontToSprite` to just above the `spriteDraw` that draws `hintImage`,
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and change the numbers. Work out why nothing happens.
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. *Right align the clock.* The clock is drawn with `fontPrint`, which cannot be
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measured. Get it against the right margin anyway. There is only one way, and
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finding it is the point.
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. *Take out the `scoreShown` check*, so `renderScore()` runs every frame
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instead. It will look identical, but it now renders the score sixty times a
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second instead of when it changes -- exactly the work `fontPrint` saves you
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when you let it. Then take out the `spriteUnload` in `renderScore` as well,
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leave it running for a minute, and watch the memory your game is using.
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. *Print a clock in milliseconds.* Change the clock line to print
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`singeGetTicks()` without the `// 1000`. It still works; it is now a string
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`fontPrint` has never seen, every frame. On a slow machine, that is the kind
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of text that costs.
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. *Use your own font.* Find a `.ttf` on your machine, copy it beside your
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script, and load it with `DIR .. "thatfont.ttf"`. Some fonts look terrible at
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18 point and fine at 48. That is the font's fault, not yours.
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=== Break It on Purpose
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Add a line reading `fontPrint(24, 60, "SCORE")` directly under
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`overlaySetResolution(discGetWidth(), discGetHeight())`, so that it runs before
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any `fontLoad` has. Singe starts and dies at once:
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----
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5:fontPrint: No font selected.
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----
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Read it the same way as any other error. `5` is the line. `fontPrint` is which
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engine call complained -- this shape of message comes from inside the engine
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rather than from Lua, so it names the function instead of the file. The rest is
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the complaint, and here it is exact: you asked it to print and there was no
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font to print with.
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You will meet this one for real in a less obvious way. Unload the selected font
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while the game is running and the next `fontPrint` says precisely the same
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thing, several hundred lines away from the `fontUnload` that caused it.
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=== What You Learned
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* `fontLoad` opens a TrueType file at one fixed point size and returns a
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handle. One size, one handle.
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* Loading a font selects it; `fontSelect` chooses between loaded fonts.
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* `fontPrint` counts in pixels and `overlayPrint` counts in character cells.
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* Colour is not an argument. `colorForeground` is a setting; set it, then draw.
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* `fontQuality` picks how letters are rasterized, and `FONT_QUALITY_BLENDED`
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is the usual answer.
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* `fontPrint` keeps what it renders, so printing the same text every frame is
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cheap; text that is different every frame is what costs.
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* `fontToSprite` renders into a sprite, which `spriteGetWidth` measures and the
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`sprite*` calls can scale and turn. Use it for that, not for speed.
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* Measuring is how you centre or right align text, because nothing else can
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tell you how wide a string will be.
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* A sprite's colour is fixed when it is made, not when it is drawn.
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* `fontUnload` frees a font; the sprites it made survive it.
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=== Next Time
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|
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Text that looks good is a long way from a menu that works. A page with a
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title, a list you move a cursor through, a slider, and a button is a hundred
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|
lines of drawing and cursor arithmetic if you build it out of the calls in this
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|
lesson. Lesson twenty does not build it out of those calls at all.
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