Is it true that it is more efficient to use PixelFormat.Bgra8888 for any case?

[BR7RKR]

Is it true that the best way to use WriteableBitmap is to convert any image (even gray scale) to PixelFormat.Bgra8888?
I've asked 2 ai agents and they both told me about PixelFormat.Bgra8888. I couldn't find any proofs in the docs or issues.
Can you help me, please?

[LaurentInSeattle]

In your C# app, grayscale images will load faster, use less bandwidth, less memory and will be processed faster.
On the GPu, less video memory and improved bandwidth as well for images and textures.
I dont exactly know what Avalonia and Skia are doing in the middle but I assume that they will handle efficiently gray scale images...
Therefore, I am almost sure that a Gray8 writeable bitmap will perform better than RGBA or ARGB for a grayscale image.

[stevemonaco]

Beware of the cost of pixel transcoding. When you lock (and unlock) the framebuffer, there will be a transcoding copy made.

See

Avalonia/src/Avalonia.Base/Media/Imaging/WriteableBitmap.cs

Lines 67 to 79 in 4e1b90b

	public ILockedFramebuffer Lock()
	{
	if (_pixelFormatMemory == null)
	return ((IWriteableBitmapImpl)PlatformImpl.Item).Lock();
	return new LockedFramebuffer(_pixelFormatMemory.Address, _pixelFormatMemory.Size,
	_pixelFormatMemory.RowBytes,
	Dpi, _pixelFormatMemory.Format, _pixelFormatMemory.AlphaFormat, () =>
	{
	using var inner = ((IWriteableBitmapImpl)PlatformImpl.Item).Lock();
	_pixelFormatMemory.CopyToRgba(inner.AlphaFormat, inner.Address, inner.RowBytes);
	});
	}

You may need to dip deeper and use Skia directly (and render via Avalonia's Skia leasing), but I'm not entirely sure if that supports the scenario efficiently either.

[BR7RKR]

Do you mean that when I lock the bitmap it gets converted to the Rgba no matter what pixel format I use? Therefore it is better to use single format for all types of pictures no matter what. Am I right?

[stevemonaco]

If you're using a WriteableBitmap that gets frequently locked with an off-format, yes. That's not necessarily the case for a Bitmap though.

And as I said, you can use SkiaSharp directly if it's a problem for you.

[LaurentInSeattle]

1 - Why is it copied ?

2 - And if absolutely necessary, why not in the original pixel format ?

[stevemonaco]

It's a copy + transcode only if the format is not supported. I assume by the renderer/compositor on the Avalonia side, but I don't know Skia / D2D well enough.

See

Avalonia/src/Skia/Avalonia.Skia/PlatformRenderInterface.cs

Lines 52 to 55 in 059bd5f

	public bool IsSupportedBitmapPixelFormat(PixelFormat format) =>
	format == PixelFormats.Rgb565
	|| format == PixelFormats.Bgra8888
	|| format == PixelFormats.Rgba8888;

[lindexi]

Reference: #20559

[BR7RKR]

Thanks for your help!