How to specify behavior of Java BufferedImage resize: need min for pixel rows instead of averaging

Question

I would like to resize a Java BufferedImage, making it smaller vertically but without using any type of averaging, so that if a pixel-row is "blank" (white) in the source image, there will be a white pixel-row in the corresponding position of the destination image: the "min" operation. The default algorithms (specified in getScaledInstance) do not allow me a fine-grained enough control. I would like to implement the following logic:

for each pixel row in the w-pixels wide destination image, d = pixel[w]
   find the corresponding j pixel rows of the source image, s[][] = pixel[j][w]
   write the new line of pixels, so that d[i] = min(s[j][i]) over all j, i

I have been reading on RescaleOp, but have not figured out how to implement this functionality -- it is admittedly a weird type of scaling. Can anyone provide me pointers on how to do this? In the worse case, I figure I can just reserve the destination ImageBuffer and copy the pixels following the pseudocode, but I was wondering if there is better way.

Answer 1

The RescaleOp methods include a parameter called RenderingHints. There is a hint called KEY_INTERPOLATION that decides the color to use when scaling an image.

If you use the value VALUE_INTERPOLATION_NEAREST_NEIGHBOR for the KEY_INTERPOLATION, Java will use the original colors, rather than using some type of algorithm to recalculate the new colors.

So, instead of white lines turning to gray or some mix of color, you'll get either white lines, or you won't get any lines at all. It all depends on the scaling factor, and if it's an even or odd row. For example, if you are scaling by half, then each 1 pixel horizontal line has at least a 50% change of appearing in the new image. However, if the white lines were two pixels in height, you'd have a 100% chance of the white line appearing.

This is probably the closest you're going to get besides writing your own scaling method. Unfortunately, I don't see any other hints that might help further.

To implement your own scaling method, you could create a new class that implements the BufferedImageOp interface, and implement the filter() method. Use getRGB() and setRGB() on the BufferedImage object to get the pixels from the original image and set the pixels on the new image.