I’ve got a shader that computes radiance texture in my DX11 renderer and I want to port it to opengl.

//this ifdef is only for the showcase, it's not in the original shader
// because  version directive must be first statement and may not be repeated
//the code is the same for both shaders thus to reduce code in this question I've added these preprocessing defines

#ifdef OPENGL

#version 460 core

#define int3 ivec3
#define uint3 uvec3
#define float2 vec2
#define float3 vec3
#define lerp(x, y, a) mix(x, y, a)
#define static //there's no static keyword in glsl

#endif

static const float g_PI = 4 * atan(1.0f);
static const float g_PI2 = 2 * g_PI;
static const float g_epsilon = 0.001f;
static const uint g_numSamples = 1024;

float3 getCubeMapTexCoord(const float2 imageSize, const float3 DTid)
{
    const float2 st = DTid.xy / imageSize;
    const float2 uv = 2.0 * float2(st.x, 1.0 - st.y) - 1.0f;

    float3 ret;
    if (DTid.z == 0)
        ret = float3(1.0, uv.y, -uv.x);
    else if (DTid.z == 1)
        ret = float3(-1.0, uv.y, uv.x);
    else if (DTid.z == 2)
        ret = float3(uv.x, 1.0, -uv.y);
    else if (DTid.z == 3)
        ret = float3(uv.x, -1.0, uv.y);
    else if (DTid.z == 4)
        ret = float3(uv.x, uv.y, 1.0);
    else if (DTid.z == 5)
        ret = float3(-uv.x, uv.y, -1.0);
    return normalize(ret);
}

void ComputeBasisVectors(const float3 N, out float3 S, out float3 T)
{
    T = cross(N, float3(0.0, 1.0, 0.0));
    T = lerp(cross(N, float3(1.0, 0.0, 0.0)), T, step(g_epsilon, dot(T, T)));

    T = normalize(T);
    S = normalize(cross(N, T));
}

float3 TangentToWorld(const float3 v, const float3 N, const float3 S, const float3 T)
{
    return S * v.x + T * v.y + N * v.z;
}

float GGX(const float roughness, const float NoH)
{
    float a2 = roughness * roughness;
    
    float denom = NoH * NoH * (a2 - 1.0) + 1.0;
    denom = g_PI * denom * denom;
    
    return roughness / denom;
}

float RadicalInverse_VdC(uint bits)
{
    bits = (bits << 16u) | (bits >> 16u);
    bits = ((bits & 0x55555555u) << 1u) | ((bits & 0xAAAAAAAAu) >> 1u);
    bits = ((bits & 0x33333333u) << 2u) | ((bits & 0xCCCCCCCCu) >> 2u);
    bits = ((bits & 0x0F0F0F0Fu) << 4u) | ((bits & 0xF0F0F0F0u) >> 4u);
    bits = ((bits & 0x00FF00FFu) << 8u) | ((bits & 0xFF00FF00u) >> 8u);
    return float(bits) * 2.3283064365386963e-10; // / 0x100000000
}

float2 SampleHammersley(const uint i, const uint samples)
{
    const float invSamples = 1.0 / float(samples);
    return float2(i * invSamples, RadicalInverse_VdC(i));
}

float3 RandomGGX(float2 random, float roughness)
{
    float a2 = roughness * roughness;
    
    float phi = g_PI2 * random.x;
    float cosTheta = sqrt((1.0 - random.y) / (1.0 + (a2 - 1.0) * random.y));
    float sinTheta = sqrt(1.0 - cosTheta * cosTheta);
    
    float3 dir;
    dir.x = cos(phi) * sinTheta;
    dir.y = sin(phi) * sinTheta;
    dir.z = cosTheta;
    return dir;
}

#ifdef OPENGL

layout(binding = 0, rgba32f) restrict writeonly uniform imageCube o_radianceMap;
layout(binding = 0) uniform samplerCube t_skybox;

layout(binding = 0) uniform Uniforms
{
    float g_perceptualRoughness;
};

#else

RWTexture2DArray<float4> o_radianceMap : register(u0);

SamplerState t_skyboxSampler : register(s0);
TextureCube t_skybox : register(t0);

cbuffer Uniforms : register(b0)
{
    float g_perceptualRoughness;
};

#endif

#ifdef OPENGL
layout(local_size_x = 32, local_size_y = 32, local_size_z = 1) in;
void main()
#else
[numthreads(32, 32, 1)]
void CSMain(uint3 gl_GlobalInvocationID : SV_DispatchThreadID)
#endif
{
    uint3 DTid = gl_GlobalInvocationID;

    float2 radianceMapSize;
    #ifdef OPENGL
    radianceMapSize = imageSize(o_radianceMap);
    #else
    float radianceElements;
    o_radianceMap.GetDimensions(radianceMapSize.x, radianceMapSize.y, radianceElements);
    #endif

    if (DTid.x >= uint(radianceMapSize.x) || DTid.y >= uint(radianceMapSize.y)) {
        return;
    }

    float2 skyboxSize;
    #ifdef OPENGL
    skyboxSize = textureSize(t_skybox, 0);
    #else
    float skyboxElements;
    t_skybox.GetDimensions(0, skyboxSize.x, skyboxSize.y, skyboxElements);
    #endif

        const float wt = 4.0 * g_PI / (6 * skyboxSize.x * skyboxSize.y);    
    const float3 N = getCubeMapTexCoord(radianceMapSize.xy, DTid);
    const float3 V = N;
    
    float3 S, T;
    ComputeBasisVectors(N, S, T);

    float3 radiance = float3(0.0f, 0.0f, 0.0f);
    float weight = 0;

    static const float MIN_PERCEPTUAL_ROUGHNESS = 0.045f;
    const float perceptualRoughness = clamp(g_perceptualRoughness, MIN_PERCEPTUAL_ROUGHNESS, 1.0);
    const float roughness = perceptualRoughness * perceptualRoughness;
    
    for (uint i = 0; i < g_numSamples; i++) {
        const float2 u = SampleHammersley(i, g_numSamples);
        const float3 H = TangentToWorld(RandomGGX(u, roughness), N, S, T);  
        const float3 L = reflect(-V, H);
        const float NoL = dot(N, L);
        
        if (NoL > g_epsilon) {
            const float NoH = max(dot(N, H), 0.0);    
            const float pdf = GGX(roughness, NoH) * 0.25;        
            const float ws = 1.0 / (g_numSamples * pdf);        
            const float mipLevel = max(0.5 * log2(ws / wt) + 1.0, 0.0);

            #ifdef OPENGL
            radiance += textureLod(t_skybox, L, mipLevel).rgb * NoL;
            #else
            radiance += t_skybox.SampleLevel(t_skyboxSampler, L, mipLevel).rgb * NoL;
            #endif
            weight += NoL;
        }
    }
    radiance /= weight;

    #ifdef OPENGL
    imageStore(o_radianceMap, ivec3(DTid), vec4(radiance, 1.0f));
    #else
    o_radianceMap[DTid] = float4(radiance, 1.0f);
    #endif
}

The shader above is the same in both applications, but without these ifdefs.
I use the same cubemap in both applications.

I’ve checked g_perceptualRoughness – values match in both applications.

The problem is the texture generated using OpenGL looks different from the dx11 one (and I’m not talking that the image is flipped) and some mipmaps have problems with the first and the last columns (I believe every mipmap has this problem but it’s not clearly visible).

Please take a look at these images:

In the third screenshot, the first column is reddish and the last bluish on the left.
I am unsure if it’s not the RenderDoc bug because when I resize the OpenGL’s texture window the problem sometimes disappears. However, it does not happen on dx’s one. It’s always good.

The C++ code looks also almost identical. Let’s start from the dx one

auto& dx = D3D::get();

D3D11_TEXTURE2D_DESC desc = {};
desc.Width = 1024;
desc.Height = 1024;
desc.MipLevels = 11;
desc.ArraySize = 6;
desc.Format = DXGI_FORMAT_R32G32B32A32_FLOAT;
desc.SampleDesc.Count = 1;
desc.Usage = D3D11_USAGE_DEFAULT;
desc.BindFlags = D3D11_BIND_UNORDERED_ACCESS;
desc.MiscFlags = D3D11_RESOURCE_MISC_TEXTURECUBE;

ComPtr<ID3D11Texture2D> prefilteredTexture;
dx.getDevice5()->CreateTexture2D(&desc, nullptr, &prefilteredTexture);

D3D11_SAMPLER_DESC samplerDesc = {
    .Filter = D3D11_FILTER_MIN_MAG_MIP_LINEAR,
    .AddressU = D3D11_TEXTURE_ADDRESS_WRAP,
    .AddressV = D3D11_TEXTURE_ADDRESS_WRAP,
    .AddressW = D3D11_TEXTURE_ADDRESS_WRAP,
    .MinLOD = 0,
    .MaxLOD = D3D11_FLOAT32_MAX
};
ComPtr<ID3D11SamplerState> sampler;
D3D::get().getDevice5()->CreateSamplerState(&samplerDesc, &sampler);

//just any skybox, only 1 mip needed, for simplicity let's say there is a skybox texture created/loaded somewhere and we can access the view using getSkyboxView() func
ComPtr<ID3D11ShaderResourceView> skyboxView = getSkyboxView();
dx.getContext4()->CSSetShaderResources(0, 1, skyboxView.GetAddressOf());

dx.getContext4()->CSSetSamplers(0, 1, sampler.GetAddressOf());

ComPtr<ID3D11ComputeShader> prefilterShader = getPrefilterShader();
dx.getContext4()->CSSetShader(computeShader.Get(), nullptr, 0);

const std::uint32_t mipLevels = 11;
const float deltaRoughness = 1.0f / glm::max(float(mipLevels - 1), 1.0f);

for (std::uint32_t level = 0, size = 1024; level < mipLevels; ++level, size /= 2) {
    const std::uint32_t numGroups = glm::max<std::uint32_t>(1, size / 32);

    D3D11_UNORDERED_ACCESS_VIEW_DESC uavDesc = {};
    uavDesc.Format = DXGI_FORMAT_R32G32B32A32_FLOAT;
    uavDesc.ViewDimension = D3D11_UAV_DIMENSION_TEXTURE2DARRAY;
    uavDesc.Texture2DArray.MipSlice = level;
    uavDesc.Texture2DArray.FirstArraySlice = 0;
    uavDesc.Texture2DArray.ArraySize = 6;

    ComPtr<ID3D11UnorderedAccessView> uav;
    D3D::get().getDevice5()->CreateUnorderedAccessView(prefilteredTexture.Get(), &uavDesc, &uav);
    dx.getContext4()->CSSetUnorderedAccessViews(0, 1, uav.GetAddressOf(), nullptr);

    //creation of the cbuffer is not important in this context
    const float roughness = level * deltaRoughness;
    roughnessBuffer.AddData(roughness);
    roughnessBuffer.Bind();

    dx.getContext4()->Dispatch(numGroups, numGroups, 6);
}

so again, to port this, I’ve just copied as a whole and changed dx functions to opengl equivalents, like this:

glEnable(GL_TEXTURE_CUBE_MAP_SEAMLESS);

GLuint prefilteredTexture;
glCreateTextures(GL_TEXTURE_CUBE_MAP, 1, &prefilteredTexture);
glTextureStorage2D(prefilteredTexture, 11, GL_RGBA32F, 1024, 1024);

GLuint sampler;
glCreateSamplers(1, &sampler);

glSamplerParameteri(sampler, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
glSamplerParameteri(sampler, GL_TEXTURE_MIN_FILTER, GL_LINEAR_MIPMAP_LINEAR);
                              
glSamplerParameteri(sampler, GL_TEXTURE_WRAP_S, GL_REPEAT);
glSamplerParameteri(sampler, GL_TEXTURE_WRAP_T, GL_REPEAT);
glSamplerParameteri(sampler, GL_TEXTURE_WRAP_R, GL_REPEAT);

//just any skybox, only 1 mip needed, for simplicity let's say there is a skybox texture created/loaded somewhere and we can access it using getSkyboxView() func
GLuint skybox = getSkybox();
glBindSampler(0, sampler);
glBindTextureUnit(0, skybox);

GLuint prefilterShader = getPrefilterShader();
glUseProgram(prefilterShader);

const std::uint32_t mipLevels = 11;
const float deltaRoughness = 1.0f / glm::max(float(mipLevels - 1), 1.0f);

for (std::uint32_t level = 0, size = 1024; level < mipLevels; ++level, size /= 2) {
    const std::uint32_t numGroups = glm::max<std::uint32_t>(1, size / 32);

    glBindImageTexture(0, prefilteredTexture, level, true, 0, GL_WRITE_ONLY, GL_RGBA32F);

    //creation of the uniform buffer is not important in this context
    const float roughness = level * deltaRoughness;
    roughnessBuffer.AddData(roughness);
    roughnessBuffer.Bind();

    glDispatchCompute(numGroups, numGroups, 6);

    glMemoryBarrier(GL_SHADER_IMAGE_ACCESS_BARRIER_BIT);
}

how can I debug this? Do I miss some configuration? Do you spot something I have to change? Where is the problem?