OK, my lack of understanding of Paint objects and how filtering works let me to make some incorrect assumptions about how straight forward this might be.
I have figured it out in the end anyway.
To do additive blending you need grey scale images that represent each channel of colour: alpha, red, green and blue.
Gimp has the functionality to separate images into these individual colour channels.
Once separated, I add them to my project resources and, as suggested above, create temporary bitmaps the same size as the screen, a paint object and a canvas for each colour and the alpha channel.
For each paint I add the corresponding grey scale image to a BitmapShader (with Mode set to CLAMP) and set it to the paint.
I also add a new PorterDuffColorFilter to each paint set to the corresponding colour, and multiply mode.
Here's what the red channel looks like:
redPaint.setShader(new BitmapShader(redTempBmp, TileMode.CLAMP,
TileMode.CLAMP));
redPaint.setColorFilter(new PorterDuffColorFilter(Color.RED,
Mode.MULTIPLY));
redPaint.setXfermode(new PorterDuffXfermode(Mode.SCREEN));
For the alpha channel (after some research) I discovered bit shifting the colour pixels (from my grey scale alpha image) into the alpha channel will result in the correct alpha levels. I found a method online to do this. here's the code:
private Paint getAlphaBitmap(int width, int height, Bitmap alphaTempBmp) {
int pixelCount = width * height;
int[] alphaPixels = new int[pixelCount];
alphaTempBmp.getPixels(alphaPixels, 0, width, 0, 0, width, height);
for(int i = 0; i < pixelCount; i++){
alphaPixels[i] = alphaPixels[i] << 8;
}
alphaTempBmp.setPixels(alphaPixels, 0, width, 0, 0, width, height);
Paint alphaPaint = new Paint();
alphaPaint.setAntiAlias(true);
alphaPaint.setXfermode(new PorterDuffXfermode(Mode.DST_IN));
return alphaPaint;
}
From this point, in my draw method I can just draw 3 rectangles, each the size of the screen using the above paints to get the desired additive blending results.
After that I draw the alphaBitmap on top and the job is done.
There is one more factor to take into consideration. When using software to split an image into separate channels, on the resulting alpha bitmap clear areas appear black. For my purposes this didn't work as I needed to draw the sprites onto the temp alpha bitmap in order to generate a screen sized alpha bitmap. This means if any images overlap then the black representing the clear parts of the image overwrite the coloured parts on the image beneath it.
So, I drew my sprites as I wanted them, and rendered them in grey scale without any alpha channel. I then created the alpha channel separately and made sure it contained only white/grey without any black. The results work quite well.
Now however, I have another problem - the code is quite slow, I hope multi-threading will sort the issue, otherwise I'm back to square 1.
EDIT: This is way to slow to use for any sort of animation. Needs to be applied using OpenGL.
