odxt's Introduction

Small S3 Texture Compression Library for Odin

(The original implementation is a port of stb_dxt.h)

Supports BC1-5 and DXT1-5

S3TC is a technique for compressing images for use as textures. Standard image compression techniques like JPEG and PNG can achieve greater compression ratios than S3TC. However, S3TC is designed to be implemented in high-performance hardware. JPEG and PNG decompress images all-at-once, while S3TC allows specific sections of the image to be decompressed independently.

S3TC is a block-based format. The image is broken up into 4x4 blocks. For non-power-of-two images that aren't a multiple of 4 in size, the other colors of the 4x4 block are taken to be black. Each 4x4 block is independent of any other, so it can be decompressed independently. (from opengl wiki)

Supported Formats

Name	Description	Premul Alpha	Compression	Texture type
BC1/DXT1	1-bit alpha / opaque	Yes	6:1 (24bit src)	Simple non-alpha
BC2/DXT2	Explicit alpha	Yes	4:1	Sharp alpha
BC2/DXT3	Explicit alpha	No	4:1	Sharp alpha
BC3/DXT4	Interpolated alpha	Yes	4:1	Gradient alpha
BC3/DXT5	Interpolated alpha	No	4:1	Gradient alpha
BC4	Interpolated greyscale	--	2:1	Gradient
BC5	Interpolated two-channel	--	2:1	Gradient

Usage

import dxt "odxt"

main :: proc() {
    image = load_image("foo.png")
    
    for bx := 0; bx < image.width; bx += 4 {
        for by := 0; by < image.height; by += 4 {
            block: [16][4]u8
            for sx in 0..<4 {
                for sy in 0..<4 {
                    x = bx + sx
                    y = by + sy
                    if x >= image.width || y > image.width {
                        continue
                    }
                    
                    index = x + y * image.width
                    block[sx + sy * 4] = {
                        image.data[index + 0],
                        image.data[index + 1],
                        image.data[index + 2],
                        image.data[index + 3],
                    }
                }
            }
            
            compressed := dxt.compress_bc1_block(block)
            
            // ...
        }
    }
}