I'm using WIC (Windows Imaging Component) to decode image files and get access to the pixel data. I'm trying to figure out the pixel order (i.e., bottom-up or top-down).
I use IWICImagingFactory::CreateDecoderFromFileName to create the decoder from which I grab the (first) frame (IWICBitmapFrameDecode). With the frame, I use GetPixelFormat and GetSize to compute a buffer size, and finally I use CopyPixels to get the decoded pixel data into my buffer.
This works fine with a variety of JPEG files, giving me pixel rows in top-down sequence, and the pixels are in BGRX order (GUID_WICPixelFormat32bppBGR).
When I try with GIF files, however, the pixel rows come in bottom-up sequence. The reported pixel format is RGBA (GUID_WICPixelFormat32bppRGBA), but the ground truth shows the channel order is BGRA (with the blue in the low byte of each 32-bit pixel, just like JPEG).
My primary question: Is there a way for me to query the top-down/bottom-up orientation of the pixel data?
I found a similar question that asked about rotation when using JPEG sources, and the answer was to query the EXIF data to know whether the image was rotated. But EXIF isn't used with GIF. So I'm wondering whether I'm supposed to assume that pixels are always bottom-up, except for ones that do have an EXIF orientation that says otherwise. Update 6/25/2020 Nope, the JPEG orientation is neutral and the GIF has no orientation information, yet MS Paint and other programs can open the files in the correct orientation.
My secondary question: What's up with the incorrect channel order (RGB/BGR) from the GIF decoder?
Not only that, the WIC documentation says that the GIF decoder should return indexes into a color table (GUID_WICPixelFormat8bppIndexed) rather than actual pixel values. Is it possible some software on my machine installed its own buggy GIF decoder that supersedes the one that comes with Windows 10?