ctt-intel-texture-compressor 0.2.0

//! BC3 (DXT5) block compression — RGBA with interpolated alpha.
//!
//! # Input format
//!
//! Expects an [`RgbaSurface`] with **`R8 G8 B8 A8` interleaved** pixel data
//! (4 bytes per pixel, little-endian RGBA). All four channels are read and
//! encoded: RGB into a BC1-style color block, and alpha into a separate
//! interpolated alpha block.
//!
//! # Output
//!
//! Each 4×4 texel block is encoded into **16 bytes** (1 byte/pixel):
//! 8 bytes for the alpha block followed by 8 bytes for the color block.

use crate::RgbaSurface;
use crate::bindings::kernel;

#[must_use]
pub fn calc_output_size(width: u32, height: u32) -> usize {
    // BC3 uses 16 bytes to store each 4×4 block, giving it an average data rate of 1 byte per pixel.
    let block_count = (width.div_ceil(4) * height.div_ceil(4)) as usize;
    block_count * 16
}

#[must_use]
pub fn compress_blocks(surface: &RgbaSurface) -> Vec<u8> {
    let output_size = calc_output_size(surface.width, surface.height);
    let mut output = vec![0u8; output_size];
    compress_blocks_into(surface, &mut output);
    output
}

/// Compresses an [`RgbaSurface`] into BC3 blocks.
///
/// The surface must contain `R8 G8 B8 A8` interleaved pixel data (4 bytes per
/// pixel). All four channels (R, G, B, A) are read and encoded.
///
/// # Panics
///
/// Panics if `blocks.len()` does not equal [`calc_output_size`] for the given
/// surface dimensions.
pub fn compress_blocks_into(surface: &RgbaSurface, blocks: &mut [u8]) {
    assert_eq!(
        blocks.len(),
        calc_output_size(surface.width, surface.height)
    );
    // SAFETY: The ISPC function does not mutate the source surface; the `*mut u8`
    // pointer type is an artifact of the C header declaration.
    let mut surface = kernel::rgba_surface {
        width: surface.width as i32,
        height: surface.height as i32,
        stride: surface.stride as i32,
        ptr: surface.data.as_ptr() as *mut u8,
    };

    unsafe {
        kernel::CompressBlocksBC3_ispc(&mut surface, blocks.as_mut_ptr());
    }
}