ImageExtensions.GetBitsPerPixel() return value always be 32

[IYinxf]

Image image = Image.FromFile(fileName); image.GetBitsPerPixel(); Even if the PixelFormat of file information is Format8bppIndexed, the return here is still 32. It is necessary for me to judge it. What should I do?

[koszeggy]

Sorry, somehow I missed this issue up to now.

As GetBitsPerPixel extracts the BPP value directly from PixelFormat (see ToBitsPerPixel) the result for Format8bppIndexed is guaranteed to be 8.

Do you still have the issue? A sample image might help; otherwise, I only can imagine the following gotchas:

• You compare different instances
• The image is a multipage bitmap (eg. TIFF) and you compare different pages
• You know that the original image is an 8bpp one but the built-in decoder loaded it as a 32bpp bitmap (eg. animated GIF, icon, or an 8bpp PNG with alpha support, for example)
• You saved/reloaded an originally 8bpp bitmap by an encoder, which turned the image a 32bpp one (eg. the PNG encoder on Windows).

You can load your image by Imaging Tools to see how the decoders used by System.Drawing treat the image. If you see Format8bppIndexed there, then GetBitsPerPixel/ToBitsPerPixel extensions must return 8.