Crop image to smallest size by removing transparent pixels in java

Question

I have a sprite sheet which has each image centered in a 32x32 cell. The actual images are not 32x32, but slightly smaller. What I\'d like to do is take a cell and crop the tr

Answer 1

This code works for me. The algorithm is simple, it iterates from left/top/right/bottom of the picture and finds the very first pixel in the column/row which is not transparent. It then remembers the new corner of the trimmed picture and finally it returns the sub image of the original image.

There are things which could be improved.

1. The algorithm expects, there is the alpha byte in the data. It will fail on an index out of array exception if there is not.

2. The algorithm expects, there is at least one non-transparent pixel in the picture. It will fail if the picture is completely transparent.

   private static BufferedImage trimImage(BufferedImage img) {
   final byte[] pixels = ((DataBufferByte) img.getRaster().getDataBuffer()).getData();
   int width = img.getWidth();
   int height = img.getHeight();
   int x0, y0, x1, y1;                      // the new corners of the trimmed image
   int i, j;                                // i - horizontal iterator; j - vertical iterator
   leftLoop:
   for (i = 0; i < width; i++) {
       for (j = 0; j < height; j++) {
           if (pixels[(j*width+i)*4] != 0) { // alpha is the very first byte and then every fourth one
               break leftLoop;
           }
       }
   }
   x0 = i;
   topLoop:
   for (j = 0; j < height; j++) {
       for (i = 0; i < width; i++) {
           if (pixels[(j*width+i)*4] != 0) {
               break topLoop;
           }
       }
   }
   y0 = j;
   rightLoop:
   for (i = width-1; i >= 0; i--) {
       for (j = 0; j < height; j++) {
           if (pixels[(j*width+i)*4] != 0) {
               break rightLoop;
           }
       }
   }
   x1 = i+1;
   bottomLoop:
   for (j = height-1; j >= 0; j--) {
       for (i = 0; i < width; i++) {
           if (pixels[(j*width+i)*4] != 0) {
               break bottomLoop;
           }
       }
   }
   y1 = j+1;
   return img.getSubimage(x0, y0, x1-x0, y1-y0);
   

   }