Question

I’m loading a big jpeg file from a url using an InputStream from a URLConnection. The goal is to get an int[] with the image data as this is more efficient than using a Bitmap for further use. There are two options here.

The first is to create a Bitmap object and to copy the results in an int[]. This works in my application but the full image is in memory twice upon loading as the image data is copied into the int[] image.

Bitmap full = BitmapFactory.decodeStream(conn.getInputStream());
full.getPixels(image, 0, width, 0, 0, width, height);

To save memory, I’m trying to perform this process in a tiled fashion using a BitmapRegionDecoder.

int block = 256;
BitmapRegionDecoder decoder = BitmapRegionDecoder.
    newInstance(conn.getInputStream(), false);
Rect tileBounds = new Rect();
// loop blocks
for (int i=0; i<height; i+=block) {
    // get vertical bounds limited by image height
    tileBounds.top = i;
    int h = i+block<height ? block : height-i;
    tileBounds.bottom = i+h;
    for (int j=0; j<width; j+=block) {
        // get hotizontal bounds limited by image width
        tileBounds.left = j;
        int w = j+block<width ? block : width-j;
        tileBounds.right = j+w;
        // load tile
        tile = decoder.decodeRegion(tileBounds, null);
        // copy tile in image
        int index = i*width + j;
        tile.getPixels(image, index, width, 0, 0, w, h);
    }
}

Technically this works and I get the full image in the int[] image. Also the tiles are seemlessly inserted into the image.

Now my problem. The second method results in an image which has some kind of strange checkerboard distortion. Pixels seem to alternate between being slightly darker or slightly lighter. BitmapRegionDecoder is supposed to support jpeg, and BitmapFactory.decodeStream has no problems. What is the problem here?

Answer 1

Found it! apparently if you feed null into decoder.decodeRegion(tileBounds, null); it returns a Bitmap with quality Bitmap.Config.RGB_565 (not sure if this is device dependant). Simply feeding it a new options set returns a Bitmap of Bitmap.Config.RGB_ARGB8888 quality. By default this preferred quality is set.

BitmapFactory.Options options = new BitmapFactory.Options();
...
// load tile
tile = decoder.decodeRegion(tileBounds, options);