singe/src/ktx2Basis.cpp

/*
 *
 *  Singe 3
 *  Copyright (C) 2006-2026 Scott Duensing <scott@kangaroopunch.com>
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License
 * as published by the Free Software Foundation; either version 3
 * of the License, or (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
 * 02110-1301, USA.
 *
 */

// Singe - KTX2 textures on the Basis Universal transcoder, behind the C interface in ktx2.h.

#include <stdint.h>
#include <string.h>
#include <SDL3/SDL.h>
#include <basisu_transcoder.h>
extern "C" {
#include "ktx2.h"
}

#define KTX2_IDENTIFIER_BYTES 12


static bool                              _ready = false;
static const uint8_t                     _identifier[KTX2_IDENTIFIER_BYTES] = { 0xAB, 0x4B, 0x54, 0x58, 0x20, 0x32, 0x30, 0xBB, 0x0D, 0x0A, 0x1A, 0x0A };


static basist::transcoder_texture_format _target(Ktx2FormatE format);


static basist::transcoder_texture_format _target(Ktx2FormatE format) {
	switch (format) {
		case KTX2_BC7:
			return basist::transcoder_texture_format::cTFBC7_RGBA;
		case KTX2_ASTC:
			return basist::transcoder_texture_format::cTFASTC_4x4_RGBA;
		case KTX2_BC3:
			return basist::transcoder_texture_format::cTFBC3_RGBA;
		case KTX2_ETC2:
			return basist::transcoder_texture_format::cTFETC2_RGBA;
		default:
			return basist::transcoder_texture_format::cTFRGBA32;
	}
}


// ===== Public =====

void ktx2Free(Ktx2ImageT *image) {
	int32_t x;

	if (image->levels != NULL) {
		for (x = 0; x < image->levelCount; x++) {
			SDL_free(image->levels[x].data);
		}
		SDL_free(image->levels);
	}
	memset(image, 0, sizeof(*image));
}


bool ktx2Is(const void *bytes, size_t size) {
	return (size >= KTX2_IDENTIFIER_BYTES) && (memcmp(bytes, _identifier, KTX2_IDENTIFIER_BYTES) == 0);
}


// Every mip level of the file's first image transcoded to the wanted format (2D, one layer, one face).
bool ktx2Transcode(const void *bytes, size_t size, Ktx2FormatE wanted, Ktx2ImageT *out) {
	basist::ktx2_transcoder           transcoder;
	basist::transcoder_texture_format format = _target(wanted);
	uint32_t                          levels;
	uint32_t                          level;

	memset(out, 0, sizeof(*out));
	if (!_ready) {
		basist::basisu_transcoder_init();
		_ready = true;
	}
	if ((size > UINT32_MAX) || !transcoder.init(bytes, (uint32_t)size) || !transcoder.start_transcoding()) {
		SDL_SetError("Not a Basis Universal KTX2 file.");
		return false;
	}
	levels = transcoder.get_levels();
	if (levels == 0) {
		SDL_SetError("KTX2 file has no images.");
		return false;
	}
	out->format     = wanted;
	out->width      = (int32_t)transcoder.get_width();
	out->height     = (int32_t)transcoder.get_height();
	out->srgb       = transcoder.is_srgb();
	out->levelCount = (int32_t)levels;
	out->levels     = (Ktx2LevelT *)SDL_calloc(levels, sizeof(Ktx2LevelT));
	if (out->levels == NULL) {
		SDL_SetError("Out of memory transcoding a texture.");
		return false;
	}
	for (level = 0; level < levels; level++) {
		basist::ktx2_image_level_info info;
		Ktx2LevelT                   *dst = &out->levels[level];
		size_t                        count;
		uint32_t                      pitch = 0;

		if (!transcoder.get_image_level_info(info, level, 0, 0)) {
			ktx2Free(out);
			SDL_SetError("KTX2 level %u is unreadable.", level);
			return false;
		}
		dst->width  = (int32_t)info.m_orig_width;
		dst->height = (int32_t)info.m_orig_height;
		if (basist::basis_transcoder_format_is_uncompressed(format)) {
			count     = (size_t)info.m_orig_width * (size_t)info.m_orig_height;
			dst->size = count * basist::basis_get_uncompressed_bytes_per_pixel(format);
			pitch     = info.m_orig_width;
		} else {
			count     = info.m_total_blocks;
			dst->size = count * basist::basis_get_bytes_per_block_or_pixel(format);
		}
		if (count > UINT32_MAX) {
			ktx2Free(out);
			SDL_SetError("KTX2 level %u is too large.", level);
			return false;
		}
		dst->data = (uint8_t *)SDL_malloc(dst->size);
		if (dst->data == NULL) {
			ktx2Free(out);
			SDL_SetError("Out of memory transcoding a texture.");
			return false;
		}
		if (!transcoder.transcode_image_level(level, 0, 0, dst->data, (uint32_t)count, format, 0, pitch, 0)) {
			ktx2Free(out);
			SDL_SetError("KTX2 level %u would not transcode.", level);
			return false;
		}
	}
	return true;
}