Skiasharp: SKImage.Encode in Bmp returns null

[ThmX]

When trying to encode an image in Bmp, it always returns null.

I tried using different values for the quality from 0 to 100, knowing that anyway it does not make sense to have a quality in bmp, I actually thought it would just ignore it.

Here's a sample that returns null.

var bitmap = SKBitmap.Decode(File.OpenRead(filename));
var image = SKImage.FromBitmap(bitmap);
var data = image.Encode(SKEncodedImageFormat.Bmp, 100); // null

Any idea how to make it work? I would like to avoid using the Bitmap.Bytes as the goal is to be able to also use other encoding format.

Thanks

[mattleibow]

Is it possible to share the image? Also, if you save it as a .png or .jpg, what happens?

[ThmX]

I tried with several images as input (bmp, png and jpg) the result is always the same.
After looking a bit more, I'm wondering whether the problem is not directly located inside the native skia library, as SkiaSharp is only binding with SkiaApi.sk_image_encode_specific(...)
Any idea?

[mattleibow]

What platform was this on? Did you try saving as a .png?

[ThmX]

I'm working on OSX and Windows, same behaviour for both.
Here's a summary using different input/ouput:

good: * -> jpg, png
bad: * -> bmp

So any input is working well, however the problem is when trying to encode in bmp. Otherwise it's working.

[mattleibow]

OK, I will check it out. bmp should work, as it is the most basic, but maybe the encoder is missing for some reason.

[ThmX]

Would you like me to create a .net solution that reproduces the problem?
(Forgot to mention earlier that I'm working with .Net Core, but it shouldn't matter as it's using the native binding anyway)

[mattleibow]

OK... Here is the issue, by default, skia compiles with it's own codecs:
https://github.com/mono/skia/blob/xamarin-mobile-bindings/src/images/SkImageEncoder.cpp

Basically JPG, PNG, WEBP.

There is an option to use the Windows/CoreGraphics encoders, but take a look at this:
https://github.com/mono/skia/blob/xamarin-mobile-bindings/src/ports/SkImageEncoder_WIC.cpp#L54-L63

Now they only do PNG and JPG. Really weird since Windows can do way more:
https://msdn.microsoft.com/en-us/library/windows/desktop/ee690311(v=vs.85).aspx I have no idea why they only have a few, so I asked: https://groups.google.com/forum/#!topic/skia-discuss/a_HNOFtuyWg

Maybe they will have an answer...

[ThmX]

I just saw the answer from Hal Canary. So I'm gonna use PNG instead.
Even though the problem is not solved for BMP, I have a pretty good solution instead.

Thanks a lot for your inquiry.

[dsoronda]

I just had issue where I needed to decode Webp image using SkiaSharp and convert it to Bmp format.
Since SkiaSharp Encode is not supported I used BmpSharp as workaround. Maybe someone will find this useful (I almost created new issue for this.)

~~~csharp
if (!(outputImage.BytesPerPixel == 3 || outputImage.BytesPerPixel == 4))
throw new Exception( $"Unsupported Bits per image ({outputImage.BytesPerPixel}) for BmpSharp" );

var bitsPerPixel = outputImage.BytesPerPixel == 4 ? BmpSharp.BitsPerPixelEnum.RGBA32 : BmpSharp.BitsPerPixelEnum.RGB24;
var bmp = new BmpSharp.Bitmap( outputImage.Width, outputImage.Height, outputImage.Bytes, bitsPerPixel );
return bmp.GetBmpBytes(flipped:true);
~~~

Disclaimer : I'm author of BmpSharp