How do ColorModels and WritableRasters work in jav

Question

When working with the BufferedImage class in Java, I usually use the constructor with parameters int width, int height, int type. For a certain application, though, I wanted an image which would store the color data using bytes in the order ARGB, which can't be done in that way (it has only TYPE_4BYTE_ABGR).

I found the following solution, which worked fine:

    WritableRaster raster = Raster.createInterleavedRaster(DataBuffer.TYPE_BYTE, width, height, 4, null);
    ColorModel colorModel = new ComponentColorModel(ColorSpace.getInstance(ColorSpace.CS_sRGB), new int[]{8,8,8,8}, true, false, ColorModel.TRANSLUCENT, DataBuffer.TYPE_BYTE);
    img = new BufferedImage(colorModel, raster, false, new Hashtable<>());

I don't understand why this works?

Though - I understand that the WritableRaster is the data structure that holds the pixel data of the picture, but past that I am lost. Which of these two objects - the Raster, or the ColorModel - determines that the pixel data is in the order RGBA? And how could I simulate any of the types in BufferedImage's (int, int, int) constructor using the (ColorModel, WritableRaster, boolean, HashTable) constructor?

Answer 1

It's the method

Raster.createInterleavedRaster(DataBuffer.TYPE_BYTE, width, height, 4, null);

...that specifies the byte order. It does so implicitly, by assuming for 4 bands, you want the band offsets to be 0, 1, 2, 3 (which corresponds to RGBA; see the source for details). For RGB color space, band 0 = Red, 1 = Green, 2 = Blue and 3 = Alpha.

If you wanted a different order, you could have used a different factory method, for instance to create a raster with ARGB order:

Raster.createInterleavedRaster(DataBuffer.TYPE_BYTE, width, height, 
                               width * 4, 4, new int[] {3, 0, 1, 2}, null);

Both of these methods will create an instance of PixelInterleavedSampleModel for you, and it's this SampleModel that really controls the sample order.

For how the BufferedImage(int, int, int) constructor works, and how you could do similar things, I think the best would be to just look at the source code for yourself. It's basically one big switch statement, where for each constant TYPE_* it creates a WritableRaster and a ColorModel similar to how you do it above.

For example:

ColorModel colorModel = ColorModel.getRGBdefault();
WritableRaster raster = colorModel.createCompatibleWritableRaster(width, height);

new BufferedImage(colorModel, raster, colorModel.isAlpahPremultiplied(), null); 

...will create an image with type TYPE_INT_ARGB (the way this reverse lookup actually works is somewhat nasty, but it works... :-)). If no corresponding type exists in BufferedImage the type will be TYPE_CUSTOM (0).