Direct3D 11 texture in ABGR format when RGBA requested - direct3d

Question

I am trying to learn how to do texture mapping in Direct3D 11.
I have successfully mapped a texture onto a quad. The problem is that Direct3D is interpreting my pixel data as ABGR, when I specifically requested DXGI_FORMAT_R8G8B8A8, and I don't understand why.
Here is the code I use to create my ID3D11Texture2D object:
UINT pixels[] = {
0xff00ff00, 0xff0000ff,
0xffff0000, 0xffffffff,
};
D3D11_SUBRESOURCE_DATA subresourceData;
subresourceData.pSysMem = pixels;
subresourceData.SysMemPitch = 8;
subresourceData.SysMemSlicePitch = 16;
D3D11_TEXTURE2D_DESC texture2dDesc;
texture2dDesc.Width = 2;
texture2dDesc.Height = 2;
texture2dDesc.MipLevels = 1;
texture2dDesc.ArraySize = 1;
texture2dDesc.Format = DXGI_FORMAT_R8G8B8A8_UNORM;
texture2dDesc.SampleDesc.Count = 1;
texture2dDesc.SampleDesc.Quality = 0;
texture2dDesc.Usage = D3D11_USAGE_DEFAULT;
texture2dDesc.BindFlags = D3D11_BIND_SHADER_RESOURCE;
texture2dDesc.CPUAccessFlags = 0;
texture2dDesc.MiscFlags = 0;
ID3D11Texture2D *texture;
device->CreateTexture2D(&texture2dDesc, &subresourceData, &texture);
The code for creating the ID3D11ShaderResourceView object:
D3D11_SHADER_RESOURCE_VIEW_DESC shaderResourceViewDesc;
memset(&shaderResourceViewDesc, 0, sizeof(D3D11_SHADER_RESOURCE_VIEW_DESC));
shaderResourceViewDesc.Format = DXGI_FORMAT_R8G8B8A8_UNORM;
shaderResourceViewDesc.ViewDimension = D3D11_SRV_DIMENSION_TEXTURE2D;
shaderResourceViewDesc.Texture2D.MipLevels = 1;
shaderResourceViewDesc.Texture2D.MostDetailedMip = 0;
ID3D11ShaderResourceView *shaderResourceView;
device->CreateShaderResourceView(texture, &shaderResourceViewDesc, &shaderResourceView);
The code for creating the ID3D11SamplerState object:
D3D11_SAMPLER_DESC samplerDesc;
memset(&samplerDesc, 0, sizeof(D3D11_SAMPLER_DESC));
samplerDesc.Filter = D3D11_FILTER_MIN_MAG_MIP_POINT;
samplerDesc.AddressU = D3D11_TEXTURE_ADDRESS_WRAP;
samplerDesc.AddressV = D3D11_TEXTURE_ADDRESS_WRAP;
samplerDesc.AddressW = D3D11_TEXTURE_ADDRESS_WRAP;
samplerDesc.MaxLOD = D3D11_FLOAT32_MAX;
samplerDesc.ComparisonFunc = D3D11_COMPARISON_NEVER;
ID3D11SamplerState *samplerState;
device->CreateSamplerState(&samplerDesc, &samplerState);
And finally the HLSL pixel shader:
Texture2D image;
SamplerState samplerState;
float4 PixelMain(float4 position: SV_POSITION, float2 texel: TEXCOORD): SV_TARGET {
return image.Sample(samplerState, texel);
}
The colors of the rendered texture are
green red
blue white
But they should be
black red
black white
And, of course, I remembered to call deviceContext->PSSetShaderResources(0, 1, &shaderResourceView); and deviceContext->PSSetSamplers(0, 1, &samplerState);
Can anyone help me understand what's going on here?

Answer 1

This is an endianness issue. Most processors you're likely to deal with store integers in "little-endian" order. Which means that an integral value of 0xAABBCCDD ends up in memory as the four bytes 0xDD, 0xCC, 0xBB, 0xAA, in that order. I know it's counter-intuitive, so I encourage you to use the Visual Studio memory window to view how your pixels array end up represented in memory.
Direct3D/your GPU does not consider a pixel as a 32-bit integer, but as four consecutive 8-bit integers, so it won't swap the bytes again when reading from the texture and will see an effective pixel value of 0xDDCCBBAA. The solution is to specify your image in a byte array with four entries per pixel, one per component. In your case:
BYTE pixels[] = {
0xff, 0x00, 0xff, 0x00, 0xff, 0x00, 0x00, 0xff,
0xff, 0xff, 0x00, 0x00, 0xff, 0xff, 0xff, 0xff,
};
Also, the SysMemSlicePitch parameter is irrelevant for 2D textures. No need to set it to 16. And an alpha value of zero does not make your pixels black. You need to turn on and play with the blending states to achieve that.