WICConvertBitmapSource + CopyPixels results in blue image - c++

Question

I'm trying to use WIC to load an image into an in-memory buffer for further processing then write it back to a file when done. Specifically:
Load the image into an IWICBitmapFrameDecode.
The loaded IWICBitmapFrameDecode reports that its pixel format is GUID_WICPixelFormat24bppBGR. I want to work in 32bpp RGBA, so I call WICConvertBitmapSource.
Call CopyPixels on the converted frame to get a memory buffer.
Write the memory buffer back into an IWICBitmapFrameEncode using WritePixels.
This results in a recognizable image, but the resulting image is mostly blueish, as if the red channel is being interpreted as blue.
If I call WriteSource to write the converted frame directly, instead of writing the memory buffer, it works. If I call CopyPixels from the original unconverted frame (and update my stride and pixel formats accordingly), it works. It's only the combination of WICConvertBitmapSource plus the use of a memory buffer (CopyPixels + WritePixels) that causes the problem, but I can't figure out what I'm doing wrong.
Here's my code.
int main() {
IWICImagingFactory *pFactory;
IWICBitmapDecoder *pDecoder = NULL;
CoInitializeEx(NULL, COINIT_MULTITHREADED);
CoCreateInstance(
CLSID_WICImagingFactory,
NULL,
CLSCTX_INPROC_SERVER,
IID_IWICImagingFactory,
(LPVOID*)&pFactory
);
// Load the image.
pFactory->CreateDecoderFromFilename(L"input.png", NULL, GENERIC_READ, WICDecodeMetadataCacheOnDemand, &pDecoder);
IWICBitmapFrameDecode *pFrame = NULL;
pDecoder->GetFrame(0, &pFrame);
// pFrame->GetPixelFormat shows that the image is 24bpp BGR.
// Convert to 32bpp RGBA for easier processing.
IWICBitmapSource *pConvertedFrame = NULL;
WICConvertBitmapSource(GUID_WICPixelFormat32bppRGBA, pFrame, &pConvertedFrame);
// Copy the 32bpp RGBA image to a buffer for further processing.
UINT width, height;
pConvertedFrame->GetSize(&width, &height);
const unsigned bytesPerPixel = 4;
const unsigned stride = width * bytesPerPixel;
const unsigned bitmapSize = width * height * bytesPerPixel;
BYTE *buffer = new BYTE[bitmapSize];
pConvertedFrame->CopyPixels(nullptr, stride, bitmapSize, buffer);
// Insert image buffer processing here. (Not currently implemented.)
// Create an encoder to turn the buffer back into an image file.
IWICBitmapEncoder *pEncoder = NULL;
pFactory->CreateEncoder(GUID_ContainerFormatPng, nullptr, &pEncoder);
IStream *pStream = NULL;
SHCreateStreamOnFileEx(L"output.png", STGM_WRITE | STGM_CREATE, FILE_ATTRIBUTE_NORMAL, true, NULL, &pStream);
pEncoder->Initialize(pStream, WICBitmapEncoderNoCache);
IWICBitmapFrameEncode *pFrameEncode = NULL;
pEncoder->CreateNewFrame(&pFrameEncode, NULL);
pFrameEncode->Initialize(NULL);
WICPixelFormatGUID pixelFormat = GUID_WICPixelFormat32bppRGBA;
pFrameEncode->SetPixelFormat(&pixelFormat);
pFrameEncode->SetSize(width, height);
pFrameEncode->WritePixels(height, stride, bitmapSize, buffer);
pFrameEncode->Commit();
pEncoder->Commit();
pStream->Commit(STGC_DEFAULT);
return 0;
}

Answer 1

The PNG encoder only supports GUID_WICPixelFormat32bppBGRA (BGR) for 32bpp as specified in PNG Native Codec official documentation. When you call it with GUID_WICPixelFormat32bppRGBA, it will not do channel switching. The pervert will just use your pixels as they were BGR, not RGB, and will not tell you there's a problem.
I don't know what you're trying to do, but in your example, you could just replace GUID_WICPixelFormat32bppRGBA by GUID_WICPixelFormat32bppBGRA in the call to WICConvertBitmapSource (and also replace the definition of the last pixelFormat variable to make sure your source code is correct, but it doesn't change anything).
PS: you can use Wic to save files, not need to create stream using another API, see my answer here: Capture screen using DirectX