Bitmap threshold quicker

Question

I am trying to write a method that will take a Bitmap and force it to a strict black and white image (no shades of grey).

I first pass the bitmap to a method that makes it greyscale using colormatrix:

public Bitmap toGrayscale(Bitmap bmpOriginal)
{        
    int width, height;
    height = bmpOriginal.getHeight();
    width = bmpOriginal.getWidth();    

    Bitmap bmpGrayscale = Bitmap.createBitmap(width, height, Bitmap.Config.RGB_565);
    Canvas c = new Canvas(bmpGrayscale);
    Paint paint = new Paint();
    ColorMatrix cm = new ColorMatrix();
    cm.setSaturation(0);

    ColorMatrixColorFilter f = new ColorMatrixColorFilter(cm);
    paint.setColorFilter(f);

    c.drawBitmap(bmpOriginal, 0, 0, paint);
    return bmpGrayscale;
}

that works nice and fast..

then i pass it to another method to force the greyscaled image to a 2 color image (black and white) this method works but obviously it is going through each pixel and that takes a long time:

public Bitmap toStrictBlackWhite(Bitmap bmp){
        Bitmap imageOut = bmp;
        int tempColorRed;
        for(int y=0; y<bmp.getHeight(); y++){
            for(int x=0; x<bmp.getWidth(); x++){
                tempColorRed = Color.red(imageOut.getPixel(x,y));
                Log.v(TAG, "COLOR: "+tempColorRed);

                if(imageOut.getPixel(x,y) < 127){
                    imageOut.setPixel(x, y, 0xffffff);
                }
                else{
                    imageOut.setPixel(x, y, 0x000000);
                }               
            }
        } 
        return imageOut;
    }

anyone know a faster more efficient way to do this?

In addition to kcoppock's answer below(which will be much faster), you'll normally get a better quality threshold if you use the image median, rather than a constant `< 127`. It's a popular method because it works with just about any image, no matter what the ratio of light/dark, and gives a pretty clear picture in almost any circumstances. There's a link [here](http://stackoverflow.com/a/8959828/752320) that shows one method, along with an additional step for threshold noise, which cleans up the image even further. The whole thing works great for me, and it's fast, too. — Geobits, Feb 15 '13 at 01:35

score 3 · Answer 1 · answered Feb 14 '13 at 23:08

3

Don't use getPixel() and setPixel().

Use getPixels() which will return a multidimensional array of all pixels. Do your operations locally on this array, then use setPixels() to set back the modified array. This will be significantly faster.

answered Feb 14 '13 at 23:08

Kevin Coppock

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score 1 · Answer 2 · edited May 23 '17 at 12:23

1

Have you tried something like converting it into a byte array (see the answer here)?

And, as I look into this, the Android reference for developers about Bitmap processing might help you as well.

edited May 23 '17 at 12:23

Community

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answered Feb 14 '13 at 19:52

DigCamara

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Bitmap threshold quicker

2 Answers2