How To Combine Two Opaque Bitmaps Into One With Alpha Channel?

I have a PNG file with transparency that I'm using as OpenGL texture. I load it in Bitmap with BitmapFactory.decodeResource, then upload it to GPU. The PNG file is quite big and in

Solution 1:

Have a look at Kevin Dion's answer to this related question. He explains how to combine 4 separate images (R, G, B and A channels) but you should be able to adapt it to work with two images.

Solution 2:

Here's a complete example:

import android.content.res.Resources;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.graphics.Canvas;
import android.graphics.ColorMatrix;
import android.graphics.ColorMatrixColorFilter;
import android.graphics.Paint;
import android.graphics.PorterDuff.Mode;
import android.graphics.PorterDuffXfermode;

publicclassImageOps {

    privatestaticfinalColorMatrixsRedToAlphaMatrix=newColorMatrix(newfloat[] {
        0, 0, 0, 0, 0,
        0, 0, 0, 0, 0,
        0, 0, 0, 0, 0,
        1, 0, 0, 0, 0});

    privatestaticfinalColorMatrixColorFiltersRedToAlphaFilter=newColorMatrixColorFilter(sRedToAlphaMatrix);

    publicstatic Bitmap composeAlpha(Bitmap target, Resources resources, int rgbDrawableId, int alphaDrawableId) {
        final BitmapFactory.Optionsoptions=newBitmapFactory.Options();          
        options.inPreferredConfig = Bitmap.Config.ARGB_8888;
        options.inScaled = false;       

        // Load RGB dataBitmaprgb= BitmapFactory.decodeResource(resources, rgbDrawableId, options);

        if (target == null) {
            // Prepare result Bitmap
            target = Bitmap.createBitmap(rgb.getWidth(), rgb.getHeight(), Bitmap.Config.ARGB_8888);
        }
        Canvasc=newCanvas(target);
        c.setDensity(Bitmap.DENSITY_NONE);

        // Draw RGB data on our result bitmap
        c.drawBitmap(rgb, 0, 0, null);

        // At this point, we don't need rgb data any more: discard!
        rgb.recycle();
        rgb = null;

        // Load Alpha dataBitmapalpha= BitmapFactory.decodeResource(resources, alphaDrawableId, options);

        // Draw alpha data on our result bitmapfinalPaintgrayToAlpha=newPaint();
        grayToAlpha.setColorFilter(sRedToAlphaFilter);
        grayToAlpha.setXfermode(newPorterDuffXfermode(Mode.DST_IN));
        c.drawBitmap(alpha, 0, 0, grayToAlpha); 

        // Don't need alpha data any more: discard!
        alpha.recycle();
        alpha = null;

        return target;
    }

}

Solution 3:

Try the following: Iterate through width * height of your two images, and use Bitmap.getPixel(x,y) on each one.

int alpha = Color.red(grayscaleBitmap.getPixel(x, y)); // grayscale, so any color will doint red = Color.red(colorBitmap.getPixel(x, y));
int green = Color.green(colorBitmap.getPixel(x, y));
int blue = Color.blue(colorBitmap.getPixel(x, y));
int mergedColor = Color.argb(alpha, red, green, blue);
// save mergedColor in an int[]

Then use Bitmap.createBitmap(int[] colors, int width, int height, Bitmap.Config config) to create your new bitmap.