[QUESTION] Support for RGB/BGR 24 bytes per pixel format?

[romen-h]

I am trying to get some pixel data into an SKImage or SKBitmap for drawing it to a canvas without having to perform an expensive copy to a 4 channel format or encoding/decoding the image. This pixel data is the same format that a System.Drawing.Bitmap with PixelFormat.Format24bppRgb uses. It seems that SKImageInfo is not able to represent this pixel format though.

Here is the line I'm using to make my SKImageInfo:

SKImageInfo info = new SKImageInfo(myImage.Width, myImage.Height, SKColorType.Rgb888x, SKAlphaType.Opaque);

This results in an SKImageInfo that has BytesPerPixel = 4.
I can't find any other color type that would match this pixel format with 3 bytes per pixel, even though this is a very common format that .bmp files store internally. (BGR vs RGB aside)

Are 24bpp RGB/BGR bitmaps not supported?
Is there a way to construct an SKImageInfo that has BytesPerPixel = 3 and its other properties with the specific values I need?

[mattleibow]

I do not believe skia supports a 24-bit color type. It supports reading 8-bit, 16-bit and 32-bit colors and a few floating point colors.

I believe pixel sizes are for optimizing memory operations with either 1, 2 or 4 bytes. 3 bytes are a bit harder to copy and things like that. You might find that you get better performance with a single load and then faster operations vs a quick load and then all operations need additional math.

I would suggest asking on the skia engine discussion site for details on this: https://groups.google.com/g/skia-discuss

[romen-h]

Well it makes a lot of sense to me that it wouldn't support 24-bit for performance reasons.
I am just trying to blit a System.Drawing.Bitmap onto an SKSurface or SKBitmap though, so I think the only way to do this is to copy pixel-by-pixel in an unsafe section from the source bytes to the destination bytes? (But I was hoping to make this fast!)

[mattleibow]

That feels like it is the only way. But I would ask in skia-discuss just in case there is some method somewhere... Effectively you have 24-bit memory and need to draw it on a 32-bit surface.

They do sometimes have a helper method lying about.

[ziriax]

You could use SIMD intrinsics to do a performant conversion?

Unfortunately you don't find a lot of examples in C#, but most of the code can be easily converted from the C++ code.

For example https://github.com/ermig1979/Simd/blob/master/src/Simd/SimdSse41BgrToBgra.cpp

Or you just interop to the native code with DllImport, maybe even using the new C# native delegates ;-)

If done well, you can really achieve C++ like SIMD performance these days in .NET!

[mattleibow]

Ah, very nice! .NET has a simd implementation that falls back to normal code. Not sure of the packages offhand.

[TRSkillman]

24 bit support would be really useful! I would love to see it added at some point. Thanks.

[fbenoit-coexya]

Even if there's no way to work on the image in 24 bits format, is there any way to save it as 24 bits losslessly?