How to manipulate raw pixel data?

[Danielku15]

I was wondering how I can access the raw pixel byte array of an Image in Skija. After loading an Image via Image.makeFromEncoded I somehow could not find a good way to convert the Image to a Bitmap. And even if I would be able to create a Bitmap there seems no accessor for the raw byte[] of the image representing the pixels. I need to do some raw pixel manipulation (for diffing purposes).

On SkiaSharp you can create a SKBitmap either by using SKBitmap.Decode() or via SKBitmap.FromImage. The raw pixels are then accessible via skBitmap.Bytes.

Is this missing in Skija?

[tonsky]

Looks like it is missing, and I already have a request to add it #72. Will add next week

[Danielku15]

Thanks for the fast reply. Digging deeper I saw that there are quite some essentials missing. My use cases would be:

1. Converting Image to Bitmap for accessing the actual image contents.
   Related SkiaSharp methods: SKBitmap.Decode(SKData), SKBitmap.FromImage(SKImage)
2. Converting Bitmap to Image for saving the bitmap again to disk after manipulations.
   Related SkiaSharp method: SKImage.FromBitmap(SKBitmap)
3. Access the binary data of a Bitmap for raw pixel manipulations. I want to do diffing of images with anti-alias detection, basically a port of PixelMatch.
   Related SkiaSharp property: SKBitmap.Bytes
4. Converting a raw pixel byte array to a Bitmap/Image for further saving. The pixels which differ are written to a raw byte array. To save these "diff" pixels as PNG to disk I need to create an image out of it.
   Related SkiaSharp method: SKImage.FromPixels(SKImageInfo, SKData)

Hope this helps as input to cover my needs 😁 .

[tonsky]

Added everything except SKBitmap.Decode (will add later). Check out https://github.com/JetBrains/skija/blob/3fda7b5b74a42b2b015bebe2031fccff631fb409/examples/scenes/src/BitmapImageScene.java for examples. From your description, you should be able to do what you set up to do. Jar version 0.89.32

[Danielku15]

Thanks a lot for the fast extensions. The changes seem to cover my usecases. The memory load might not be optimal because the read/installPixel workflows usually result in a full copy of the raw buffer instead sharing buffers or accessing the raw buffer of a bitmap for in-place modifications.

In my concrete case it shouldn't be a problem with the memory consumption.

It also seems with my rework that I do not have a direct need for SKBitmap.Decode anymore. I work now rather on Image base and just for the diffing I do a one time load of the raw pixels via Bitmap.makeFromImage(img).readPixels(). But I leave it up to you to still implement it. From my side the issue can be closed 😉

[tonsky]

Will see if direct memory access is possible via ByteBuffers

[Danielku15]

https://docs.oracle.com/javase/8/docs/technotes/guides/jni/spec/functions.html#NewDirectByteBuffer
This should do the job 😉 . Beside that Skia has some built-in conversion functions which work based on the underlying pixel buffer. e.g. SkImage.MakeFromBitmap

[cqjjjzr]

What about manipulating raw pixels in a raster based SkSurface? on SkiaSharp we have ´SkSurface.peekPixels´ which returns a SkPixmap,and no copying is involved. So how to do so in Skija?
Thanks.

[tonsky]

@cqjjjzr if you need it, please open a new issue