Unexpected rowpitch from WIC texture to DX11 texture

Question

I am confused as to why I am getting 128 bytes for scan-lines/rowpitch rather than the 8 I am specifying. The BMP in question being loaded is 4 pixels, red top left, green top right, blue bottom left, yellow bottom right.

Code is as follows:

ID3D11Resource *g_pOverlay_Staging = nullptr;
IWICImagingFactory* WICFactory = nullptr;
IWICBitmapDecoder * WICBitmapDecoder = nullptr;
IWICBitmapFrameDecode * WICBitmapFrame = nullptr;
WICPixelFormatGUID pixelFormat;
HR(CoCreateInstance(CLSID_WICImagingFactory, nullptr, CLSCTX_INPROC_SERVER, __uuidof(IWICImagingFactory), (LPVOID*)&WICFactory));

HR(WICFactory->CreateDecoderFromFilename(L"province_map.bmp", nullptr, GENERIC_READ, WICDecodeMetadataCacheOnDemand, &WICBitmapDecoder));

HR(WICBitmapDecoder->GetFrame(0,&WICBitmapFrame));

HR(WICBitmapFrame->GetPixelFormat(&pixelFormat));

DXGI_FORMAT format;

if (pixelFormat == GUID_WICPixelFormat24bppBGR){
    std::wostringstream outs2;
    outs2 << "pixelformat = GUID_WICPixelFormat24bppBGR" << std::endl;
    OutputDebugString(outs2.str().c_str());
    format = DXGI_FORMAT_R8G8B8A8_UNORM;
}

IWICFormatConverter * FC = nullptr;
WICFactory->CreateFormatConverter(&FC);
HR(FC->Initialize(WICBitmapFrame, GUID_WICPixelFormat32bppRGBA, WICBitmapDitherTypeErrorDiffusion, 0, 0, WICBitmapPaletteTypeCustom));
std::unique_ptr<uint8_t[]> temp(new uint8_t[16]);
HR(FC->CopyPixels(0, 8, 16, temp.get()));

D3D11_TEXTURE2D_DESC desc;
desc.Width = 2;
desc.Height = 2;
desc.MipLevels = 1;
desc.ArraySize = 1;
desc.Format = format;
desc.SampleDesc.Count = 1;
desc.SampleDesc.Quality = 0;
desc.Usage = D3D11_USAGE_STAGING;
desc.BindFlags = 0;
desc.CPUAccessFlags = D3D11_CPU_ACCESS_WRITE | D3D11_CPU_ACCESS_READ;
desc.MiscFlags = 0;

D3D11_SUBRESOURCE_DATA initData;
initData.pSysMem = temp.get();
initData.SysMemPitch = 8;
initData.SysMemSlicePitch = 16;
ID3D11Texture2D* tex = nullptr;
HR(d3d11DeviceInterface->CreateTexture2D(&desc, &initData, &tex));
g_pOverlay_Staging = tex;

D3D11_MAPPED_SUBRESOURCE mapped_subresource;
HR(d3d11DeviceContextInterface->Map(g_pOverlay_Staging, 0, D3D11_MAP_READ_WRITE, 0, &mapped_subresource));
uint8_t * v = reinterpret_cast<uint8_t*>(mapped_subresource.pData);
unsigned int x = 0;

while (x < 256 ) {
std::bitset<8> one(v[x]);
std::bitset<8> two(v[x+1]);
std::bitset<8> three(v[x+2]);
std::bitset<8> four(v[x + 3]);
std::wostringstream outs1;
outs1 << "x[" << x << "] = " << one << " x[" << x + 1 << "] = " << two << " x[" << x + 2 << "] = " << three << " x[" << x + 3 << "] = " << four << std::endl;
OutputDebugString(outs1.str().c_str());
x += 4;
}

Output is:

pixelformat = GUID_WICPixelFormat24bppBGR
x[0] = 11111111 x[1] = 00000000 x[2] = 00000000 x[3] = 11111111
x[4] = 00000000 x[5] = 11111111 x[6] = 00000000 x[7] = 11111111
x[8] = 00000000 x[9] = 00000000 x[10] = 00000000 x[11] = 00000000
x[12] = 00000000 x[13] = 00000000 x[14] = 00000000 x[15] = 00000000
x[16] = 00000000 x[17] = 00000000 x[18] = 00000000 x[19] = 00000000
x[20] = 00000000 x[21] = 00000000 x[22] = 00000000 x[23] = 00000000
x[24] = 00000000 x[25] = 00000000 x[26] = 00000000 x[27] = 00000000
x[28] = 00000000 x[29] = 00000000 x[30] = 00000000 x[31] = 00000000
x[32] = 00000000 x[33] = 00000000 x[34] = 00000000 x[35] = 00000000
x[36] = 00000000 x[37] = 00000000 x[38] = 00000000 x[39] = 00000000
x[40] = 00000000 x[41] = 00000000 x[42] = 00000000 x[43] = 00000000
x[44] = 00000000 x[45] = 00000000 x[46] = 00000000 x[47] = 00000000
x[48] = 00000000 x[49] = 00000000 x[50] = 00000000 x[51] = 00000000
x[52] = 00000000 x[53] = 00000000 x[54] = 00000000 x[55] = 00000000
x[56] = 00000000 x[57] = 00000000 x[58] = 00000000 x[59] = 00000000
x[60] = 00000000 x[61] = 00000000 x[62] = 00000000 x[63] = 00000000
x[64] = 00000000 x[65] = 00000000 x[66] = 00000000 x[67] = 00000000
x[68] = 00000000 x[69] = 00000000 x[70] = 00000000 x[71] = 00000000
x[72] = 00000000 x[73] = 00000000 x[74] = 00000000 x[75] = 00000000
x[76] = 00000000 x[77] = 00000000 x[78] = 00000000 x[79] = 00000000
x[80] = 00000000 x[81] = 00000000 x[82] = 00000000 x[83] = 00000000
x[84] = 00000000 x[85] = 00000000 x[86] = 00000000 x[87] = 00000000
x[88] = 00000000 x[89] = 00000000 x[90] = 00000000 x[91] = 00000000
x[92] = 00000000 x[93] = 00000000 x[94] = 00000000 x[95] = 00000000
x[96] = 00000000 x[97] = 00000000 x[98] = 00000000 x[99] = 00000000
x[100] = 00000000 x[101] = 00000000 x[102] = 00000000 x[103] = 00000000
x[104] = 00000000 x[105] = 00000000 x[106] = 00000000 x[107] = 00000000
x[108] = 00000000 x[109] = 00000000 x[110] = 00000000 x[111] = 00000000
x[112] = 00000000 x[113] = 00000000 x[114] = 00000000 x[115] = 00000000
x[116] = 00000000 x[117] = 00000000 x[118] = 00000000 x[119] = 00000000
x[120] = 00000000 x[121] = 00000000 x[122] = 00000000 x[123] = 00000000
x[124] = 00000000 x[125] = 00000000 x[126] = 00000000 x[127] = 00000000
x[128] = 00000000 x[129] = 00000000 x[130] = 11111111 x[131] = 11111111
x[132] = 11111111 x[133] = 11111111 x[134] = 00000000 x[135] = 11111111
x[136] = 00000000 x[137] = 00000000 x[138] = 00000000 x[139] = 00000000
x[140] = 00000000 x[141] = 00000000 x[142] = 00000000 x[143] = 00000000
x[144] = 00000000 x[145] = 00000000 x[146] = 00000000 x[147] = 00000000
x[148] = 00000000 x[149] = 00000000 x[150] = 00000000 x[151] = 00000000
x[152] = 00000000 x[153] = 00000000 x[154] = 00000000 x[155] = 00000000
x[156] = 00000000 x[157] = 00000000 x[158] = 00000000 x[159] = 00000000
x[160] = 00000000 x[161] = 00000000 x[162] = 00000000 x[163] = 00000000
x[164] = 00000000 x[165] = 00000000 x[166] = 00000000 x[167] = 00000000
x[168] = 00000000 x[169] = 00000000 x[170] = 00000000 x[171] = 00000000
x[172] = 00000000 x[173] = 00000000 x[174] = 00000000 x[175] = 00000000
x[176] = 00000000 x[177] = 00000000 x[178] = 00000000 x[179] = 00000000
x[180] = 00000000 x[181] = 00000000 x[182] = 00000000 x[183] = 00000000
x[184] = 00000000 x[185] = 00000000 x[186] = 00000000 x[187] = 00000000
x[188] = 00000000 x[189] = 00000000 x[190] = 00000000 x[191] = 00000000
x[192] = 00000000 x[193] = 00000000 x[194] = 00000000 x[195] = 00000000
x[196] = 00000000 x[197] = 00000000 x[198] = 00000000 x[199] = 00000000
x[200] = 00000000 x[201] = 00000000 x[202] = 00000000 x[203] = 00000000
x[204] = 00000000 x[205] = 00000000 x[206] = 00000000 x[207] = 00000000
x[208] = 00000000 x[209] = 00000000 x[210] = 00000000 x[211] = 00000000
x[212] = 00000000 x[213] = 00000000 x[214] = 00000000 x[215] = 00000000
x[216] = 00000000 x[217] = 00000000 x[218] = 00000000 x[219] = 00000000
x[220] = 00000000 x[221] = 00000000 x[222] = 00000000 x[223] = 00000000
x[224] = 00000000 x[225] = 00000000 x[226] = 00000000 x[227] = 00000000
x[228] = 00000000 x[229] = 00000000 x[230] = 00000000 x[231] = 00000000
x[232] = 00000000 x[233] = 00000000 x[234] = 00000000 x[235] = 00000000
x[236] = 00000000 x[237] = 00000000 x[238] = 00000000 x[239] = 00000000
x[240] = 00000000 x[241] = 00000000 x[242] = 00000000 x[243] = 00000000
x[244] = 00000000 x[245] = 00000000 x[246] = 00000000 x[247] = 00000000
x[248] = 00000000 x[249] = 00000000 x[250] = 00000000 x[251] = 00000000
x[252] = 00000000 x[253] = 00000000 x[254] = 00000000 x[255] = 00000000

Answer 1

This is expected. After you've uploaded texture memory to the GPU, if you Map it back for read, there's no guarantee that it will come back with the same pitch that you uploaded it as. This is because on upload, the driver re-lays out memory in a way that's optimal for rendering. When you bring it back to the CPU, the driver efficiently converts it back to linear format, usually in a way that reflects various properties of the texture cache or page size.