vigneshvg
commented
1 year ago Hello @vigneshvg --
I was doing some internal testing with a teammate and we noticed that decoded Bitmap objects coming out of AvifDecoder.decode() are flagged as premultiplied
Can you elaborate a bit more on what exactly you mean by "Bitmaps are flagged as premultiplied"? Because the bitmap is an input parameter that's taken by the AvifDecoder.decode() function. The JNI wrapper itself does not do anything more to the bitmap other than to copy pixels. Depending on the configuration of the bitmap, the alpha plane should be handled correctly. For ARGB_8888 and RGBA_F16, the alpha plane is not premultiplied.
I have tried loading images with alpha plane several times before and it has worked as intended. Do you know which type of bitmaps you were testing? If you can share a sample image that fails to render properly, i can take a further look. :)
(as all Bitmaps must be in order to be drawn by Android draw calls), but in the guts of the JNI
decode()function, we are just using the default value ofavifRGBImage.alphaPremultiplied, which isAVIF_FALSE. This results in a Bitmap which contains the actual unpremultiplied pixels in them, yet the Bitmap itself is flagged as premultiplied. This causes incorrect blending.If I'm right (please correct me if not), there are two solutions to this:
- Legislate that all Bitmap data coming out of the Java AvifDecoder is actually premultiplied. Android wants this anyway, so it'd be as simple as adding: rgb_image.alphaPremultiplied = AVIF_TRUE;
... after the call to
avifRGBImageSetDefaults().
I don't think this assumption is true, because in case of ARGB_8888 and RGBA_F16 bitmaps, the alpha channel is not pre-multiplied.
- Expose the
alphaPremultipliedproperty in some way to Java, so consumers of this library can set it to true. I don't see a ton of value here of it ever being false, but if we're going for completeness and someone simply wants to query pixel data after decode or do post-processing before handing it over to Android to draw, that would also be fine.
This is certainly possible (and quite easy to do). I have an updated API coming up for the JNI (changes in my local branch here: https://github.com/vigneshvg/libavif/tree/cl_animated_tree). That provides better access into the decoder (proper return codes, animated images, etc). We can add a function there to expose the premultiplied value if that will help.
But in general, Android apps using the JNI API via the Bitmap class do not care about that level of detail and only want the right pixels in the Bitmap so that it can be rendered. If they want access to the raw pixels (for post-processing or whatever), then the JNI API as it stands is not a good tool for that use-case. For that, we would have to expose other functions which return the raw pixel values rather than copying them into an android.graphics.Bitmap.
Thoughts?
joedrago
Garaks
Hello @vigneshvg --
I was doing some internal testing with a teammate and we noticed that decoded Bitmap objects coming out of AvifDecoder.decode() are flagged as premultiplied (as all Bitmaps must be in order to be drawn by Android draw calls), but in the guts of the JNI
decode()function, we are just using the default value ofavifRGBImage.alphaPremultiplied, which isAVIF_FALSE. This results in a Bitmap which contains the actual unpremultiplied pixels in them, yet the Bitmap itself is flagged as premultiplied. This causes incorrect blending.If I'm right (please correct me if not), there are two solutions to this:
Legislate that all Bitmap data coming out of the Java AvifDecoder is actually premultiplied. Android wants this anyway, so it'd be as simple as adding:
rgb_image.alphaPremultiplied = AVIF_TRUE;
... after the call to
avifRGBImageSetDefaults().alphaPremultipliedproperty in some way to Java, so consumers of this library can set it to true. I don't see a ton of value here of it ever being false, but if we're going for completeness and someone simply wants to query pixel data after decode or do post-processing before handing it over to Android to draw, that would also be fine.Thoughts?