Loading a DDS file DXT1 DXT3 DXT5 opengl

dds_texture_loader.c

GLuint texture_loadDDS(const char* path) {
	
	FILE* f;
	f = fopen(path, "rb");
	if(f == 0)
		return 0;
	
	// make sure this is a dds
	char filecode[4];
	fread(&filecode, 1, 4, f);
	if(strncmp(filecode, "DDS ", 4) != 0) {
		fclose(f);
		return 0;
	}
	
	unsigned char header[124];
	fread(&header, 1, 124, f);
	
	// different depending on byte arrangement/endianess, this is the windows version
	unsigned int width = (header[8]) | (header[9] << 8) | (header[10] << 16) | (header[11] << 24);
	unsigned int height = (header[12]) | (header[13] << 8) | (header[14] << 16) | (header[15] << 24);
	unsigned int linearSize = (header[16]) | (header[17] << 8) | (header[18] << 16) | (header[19] << 24);
	unsigned int mipMapCount = (header[24]) | (header[25] << 8) | (header[26] << 16) | (header[27] << 24);
	
	// a 4-byte array of chars
	char* fourCC = (char*)&(header[80]);
	
	// 'the sum of all the mipmap's byte-size' is never greater than 'two times the biggest mipmap byte-size'
	unsigned int buffer_size = (mipMapCount > 1 ? linearSize + linearSize : linearSize);
	unsigned char* buffer = malloc(buffer_size * sizeof(unsigned char));
	if(buffer == 0) {
		// Handle OOM
		return 0;
	}
	
	fread(buffer, 1, buffer_size, f);
	
	fclose(f);
	
	// decide byte format and block size from type
	// probably should check if the EXT is available
	unsigned int blockSize;
	unsigned int format;
	if(strncmp(fourCC, "DXT1", 4) == 0) {
		format = GL_COMPRESSED_RGBA_S3TC_DXT1_EXT;
		blockSize = 8;
	} else if(strncmp(fourCC, "DXT3", 4) == 0) {
		format = GL_COMPRESSED_RGBA_S3TC_DXT3_EXT;
		blockSize = 16;
	} else if(strncmp(fourCC, "DXT5", 4) == 0) {
		format = GL_COMPRESSED_RGBA_S3TC_DXT5_EXT;
		blockSize = 16;
	} else {
		// unhandled type
		free(buffer);
		return 0;
	}
	
	// glGenTextures() CAN return 0 on error, see docs - probably won't happen unless you are bad
	GLuint tid;
	glGenTextures(1, &tid);
	if(tid == 0) {
		free(buffer);
		return 0;
	}
	
	glBindTexture(GL_TEXTURE_2D, tid);
	
	// dds gives you the wrong amount of mipMaps, MAX_LEVEL 0 = 1 mipmap, MAX_LEVEL 1 = 2 mipmaps
	// thusly just minus one - simple off-by-one array fix
	glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_BASE_LEVEL, 0);
	glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAX_LEVEL, mipMapCount-1);
	glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
	glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR_MIPMAP_LINEAR);
	glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_REPEAT);
	glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_REPEAT);
	
	unsigned int offset = 0;
	for (unsigned int level=0; level<mipMapCount && (width || height); level++) {
		// ignore mipmaps you will never see, save 4*4*4 + 2*2*4 + 1*1*4 data I guess
		// if you don't at least ignore the 0*0 mipmaps, you will 0/2
		// dds can have mipmaps that aren't rectangular, thusly 256*128 and 128*64 and 1*0 can be levels
		if(width <= 4 || height <= 4) {
			// -1 because this iteration of level is the next, which we don't use
			glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAX_LEVEL, level-1);
			break;
		}
		
		// the 3's aren't magic numbers
		unsigned int size = ((width+3)/4) * ((height+3)/4) * blockSize;
		glCompressedTexImage2D(GL_TEXTURE_2D, level, format, width, height, 0, size, buffer + offset);
		
		// offset is current byte offset from mip to map, as data is stored linearly
		offset += size;
		
		// assuming pow2 mipmaps
		width = width/2;
		height = height/2;
	}
	
	glBindTexture(GL_TEXTURE_2D, 0);
	
	free(buffer);
	return tid;
}