Newbie question on getting Pixel Format from an image

[iMrShadow]

Hello! Is there a way to know what pixel / color format is used in the original image?

For example, I want to know whether the original pixel format is in
Format32bppArgb
Format16bppArgb1555
Format16bppRgb555
Format16bppRgb565
Format24bppRgb
Format8bppIndexed
and others.
But I want their equivalent counter parts from this table:

I am working with .png, .jpg, .tiff, .bmp and .tga, and then converting them into .dds format.

[antonfirsov]

Note that System.Drawing's Image.PixelFormat doesn't necessarily represent the format used in the "original image", if you mean the encoded file by that. While it's correct for a PNG, note that JPEG stores images in YCbCr color space, meaning if you don't ask for a specific format when decoding, the outcome, new Bitmap("img.jpg").PixelFormat == PixelFormat.Format24bppRgb is not really accurate. Long story short Image.PixelFormat is the format System.Drawing decides to internally represent your image in when you are not asking for a specific format at decoding, and not the absolute thruth about the source image.

ImageSharp will also pick an arbitrary pixel format when using the non-generic Image.Load(), but I don't think that information is useful in any way for you. There is functionality to acquire information/metadata about the original image format if that is what you are after:

// The sample is using 3.1.* API
ImageInfo info = Image.Identify("my.png");
PixelTypeInfo pixelType = info.PixelType;
Console.WriteLine($"bpp: {pixelType.BitsPerPixel}");

// There is no way to get more information from pixelType at the moment,
// however you can observe format specific information:
if (info.Metadata.TryGetPngMetadata(out PngMetadata pngMetadata))
{
    // pngMetadata has plenty of properties
}

and then converting them into .dds format.

If you are resaving the image anyways, I wonder what do you need the pixel format information for?

[iMrShadow]

Thanks for the answer! The pixel format from System.Drawing was used in old code and I originally thought I needed it, but at the end I didn't.
The idea was displaying image data (width, length, depth and others) on the screen for information.

I have another question, if I may ask. I want to know if an image is transparent/has an alpha channel. However, when I use ImageInfo and extract the PixelFormat.AlphaRepresentation, that is always null (even for images which are transparent). That seems like a strange behaviour, unless I am misunderstanding something

[antonfirsov]

Unfortunately, filling AlphaRepresentation (or any values other than BitsPerPixel) on Image.Identify is not yet implemented. You need to check the format-specific metadata to figure those things out.

cc @JimBobSquarePants in case you want to put this to the V4 feature list.

[JimBobSquarePants]

I’m toying with the idea of dropping it for decoding and making it representative of pixel formats only since we have the per-format metadata. I want to do some normalisation there also.

[antonfirsov]

It is harder to code against from user POV. One needs to iterate through all the arbitrary properties of all format metadatas.

[JimBobSquarePants]

Someone would need to contribute this code then. We'd have to go through each format and assign the corresponding properties. I dread to think what that is like for Tiff.