unable to transparent pixels of a bitmap image

问题

I am working with bitmap images whose transparent parts are colored in magenta (in some languages it is possible to set a color as transparent). I try to transparent pixels which are in magenta in the original bitmap image.

I load the bitmap from SD-card:

            Bitmap bitmap = BitmapFactory.decodeFile(myImagePath);

copy it to another bitmap to make it mutable:

            Bitmap bitmap2 = bitmap.copy(Bitmap.Config.ARGB_8888,true);

Then scan it pixel by pixel to find pixels in magenta and try to change their transparency.

            for(int x=0;x<bitmap2.getWidth();x++){
                for(int y=0;y<bitmap2.getHeight();y++){
                    if(bitmap2.getPixel(x, y)==Color.rgb(0xff, 0x00, 0xff))
                    {
                        int alpha = 0x00;
                        bitmap2.setPixel(x, y , Color.argb(alpha,0xff,0xff,0xff));  // changing the transparency of pixel(x,y)
                    }                           
                }
            }

But those pixels which I expect to become transparent are converted to black. By changing the alpha, I found that the final color varies from the mentioned color in argb() (without mentioning the alpha) to black. For instance, Color.argb(0xff,0xff,0xff,0xff) gets white, Color.argb(0x80,0xff,0xff,0xff) gets gray and Color.argb(0x00,0xff,0xff,0xff) gets black.

I don't undrestand what's wrong.

Could it be possible that there is no alpha channel and I should first set/define it? if yes, how?

EDIT1:

According to the comment of Der Gol...lum I have modified my code:

    Paint mPaint = new Paint();
    mPaint.setAlpha(0);
    mPaint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.DST_OUT));
    mPaint.setAntiAlias(true);

        Bitmap bitmap = BitmapFactory.decodeFile(myBackImagePath).copy(Bitmap.Config.ARGB_8888 , true);
        Canvas canvas = new Canvas(bitmap);
        canvas.drawBitmap(bitmap, 0, 0, mPaint);

        if(bitmap.getPixel(0, 0)==Color.rgb(0xff, 0x00, 0xff))
        {
            for(int x=0;x<bitmap.getWidth();x++){
                for(int y=0;y<bitmap.getHeight();y++){
                    if(bitmap.getPixel(x, y)==Color.rgb(0xff, 0x00, 0xff))
                    {
                        bitmap.setPixel(x, y,Color.TRANSPARENT);
                    }                           
                }
            }

But the result is more or less the same. Using different PorterDuffModes causes either transparency of the entire bitmap or make the targeted pixels black:

Would anybody have any idea?

回答1:

I could finally find the problem. My png images had no alpha channel or maybe their alpha channel were not activated. what I did to solve this problem is to add:

         bitmap.setHasAlpha(true);

and it works how I expected.

来源：https://stackoverflow.com/questions/31721453/unable-to-transparent-pixels-of-a-bitmap-image

标签

android

bitmap

alpha-transparency