如何将两个不透明的位图组合成一个带alpha通道的位图？

Question

我有一个透明的PNG文件,我用作OpenGL纹理.我在加载它Bitmap用BitmapFactory.decodeResource,然后把它上传到GPU.

PNG文件相当大,为了减少APK大小,我试图使用两个JPG - 一个使用RGB数据,另一个使用alpha通道(灰度).

如何将两个JPG组合在一个Bitmap具有Alpha通道的对象中？我尝试加载alpha通道Bitmap.Config.ALPHA_8,然后使用Canvas但是没有运气到目前为止将它们绘制在彼此之上.

Answer 1

看看Kevin Dion对这个相关问题的回答.他解释了如何组合4个单独的图像(R,G,B和A通道),但您应该能够使其适应两个图像.

Answer 2

这是一个完整的例子：

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;

public class ImageOps {

    private static final ColorMatrix sRedToAlphaMatrix = new ColorMatrix(new float[] {
        0, 0, 0, 0, 0,
        0, 0, 0, 0, 0,
        0, 0, 0, 0, 0,
        1, 0, 0, 0, 0});

    private static final ColorMatrixColorFilter sRedToAlphaFilter = new ColorMatrixColorFilter(sRedToAlphaMatrix);

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

        // Load RGB data
        Bitmap rgb = BitmapFactory.decodeResource(resources, rgbDrawableId, options);

        if (target == null) {
            // Prepare result Bitmap
            target = Bitmap.createBitmap(rgb.getWidth(), rgb.getHeight(), Bitmap.Config.ARGB_8888);
        }
        Canvas c = new Canvas(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 data
        Bitmap alpha = BitmapFactory.decodeResource(resources, alphaDrawableId, options);

        // Draw alpha data on our result bitmap
        final Paint grayToAlpha = new Paint();
        grayToAlpha.setColorFilter(sRedToAlphaFilter);
        grayToAlpha.setXfermode(new PorterDuffXfermode(Mode.DST_IN));
        c.drawBitmap(alpha, 0, 0, grayToAlpha); 

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

        return target;
    }

}