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Writing to Compressed Textures

In general it's not possible to use a block-compressed texture as a render target or as a compute shader output. Instead you have to either: Alias the block compressed texture with an uncompressed texture where each texel corresponds to a block, or to output the compressed blocks to an uncompressed texture buffer, and then copy the compressed blocks from that intermediate memory location to the final compressed texture.

Each of the graphics APIs expose this functionality in a different way. This document explains the options available under the following APIs:

• Direct3D
• Vulkan
• Metal
• OpenGL

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// Returns p bit that results in the lowest RGB quantization error.
inline int quantize_rgb_bc7_mode6(float r, float g, float b, float * out_qr, float * out_qg, float * out_qb) {
    float qr = 2 * trunc(r * 127.5f);
    float qg = 2 * trunc(g * 127.5f);
    float qb = 2 * trunc(b * 127.5f);

    float dr = 255 * r - qr;
    float dg = 255 * g - qg;
    float db = 255 * b - qb;

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template <typename T>
struct Compare {
    T lambda;

#if _MSC_VER || __APPLE__
    static int compare(void * cmp, const void * a, const void * b)
#else
    static int compare(const void * a, const void * b, void * cmp)
#endif
    {

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// This code is in the public domain -- Ignacio Castaño <[email protected]>

#include "Sphere.h"
#include "Vector.inl"
#include "Box.inl"

#include <float.h> // FLT_MAX

const float radiusEpsilon = 1e-4f;

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// Emulating gathers using loads and permutevar8. This made the entire compressor about 15% faster. Both methods require AVX2.

// Load 4 uint8 per lane.
__m256i packedClusterIndex = _mm256_load_si256((__m256i *)&s_fourCluster[i]);

if (count <= 8) {
    // Load r_sat in one register:
    Wide8 r07 = load8(r_sat);
    Wide8 g07 = load8(g_sat);
    Wide8 b07 = load8(b_sat);

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    Wide8 besterror8 = broadcast8(FLT_MAX);
    Vector3_Wide8 beststart8;
    Vector3_Wide8 bestend8;

    // check all possible clusters for this total order
    for (int i = 0; i < s_fourClusterTotal[count - 1]; i += 8)
    {
        //uint c0 = s_fourCluster[i].c0;
        //uint c1 = s_fourCluster[i].c1;
        //uint c2 = s_fourCluster[i].c2;

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rgbcx v1.12
icbc v1.0
                 RMSE           PSNR            Time

Kodak/Waterloo Image Set:

rgbcx-0          8.128142       29.930977       0.163412
rgbcx-1          8.041399       30.024172       0.182219
rgbcx-2          8.198924       29.855667       0.213994
rgbcx-3          8.005721       30.062794       0.242393

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Perfect Quantization of DXT endpoints
-------------------------------------

One of the issues that affect the quality of most DXT compressors is the way floating point colors are rounded.

For example, stb_dxt does:

    max16 =  (unsigned short)(stb__sclamp((At1_r*yy - At2_r*xy)*frb+0.5f,0,31) << 11);
    max16 |= (unsigned short)(stb__sclamp((At1_g*yy - At2_g*xy)*fg +0.5f,0,63) << 5);
    max16 |= (unsigned short)(stb__sclamp((At1_b*yy - At2_b*xy)*frb+0.5f,0,31) << 0);

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                 RMSE           PSNR            Time

Kodak/Waterloo Image Set:

stb              8.202766       29.851597       0.258041
stb-hq           8.009301       30.058910       0.284019
nvtt-fast        8.089954       29.971882       0.445670

nvtt             7.616215       30.496019       6.806233
nvtt-hq          7.562366       30.557650       13.081200

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// This code is in the public domain -- Ignacio Castaño <[email protected]>

#include "Sphere.h"
#include "Vector.inl"
#include "Box.inl"

#include <float.h> // FLT_MAX

using namespace nv;