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I'm making a lighting system right now, and the last part I have to do is to invert the alpha channel on the final light map. My previous code that used to work was

DataBufferByte buf = (DataBufferByte)lightmap.getRaster().getDataBuffer();
    byte[] values = buf.getData();
    for(int i = 0; i < values.length; i += 4) values[i] = (byte)(values[i] ^ 0xff);

But this code threw this "java.lang.ClassCastException: java.awt.image.DataBufferInt cannot be cast to java.awt.image.DataBufferByte" on the first line. So, I changed it to

DataBufferInt buf = (DataBufferInt)lightmap.getRaster().getDataBuffer();
    int[] values = buf.getData();
    for(int i = 0; i < values.length; i += 4) values[i] = (int)(values[i] ^ 0xff);

but this doesn't actually invert the alpha, but seems to invert the color between the hue and black? I know that this works

for(int i = 0; i < lightmap.getWidth(); i++) {
        for(int j = 0; j < lightmap.getHeight(); j++) {
            lightmap.setRGB(i,j,(lightmap.getRGB(i, j) ^ 0xFF000000));
        }
    }

But testing this, it brings my framerate down to 15 fps.

So I guess my question is this: Either how can I fix this, or is there a fast way to invert the alpha values of a bufferedimage?

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1 Answer 1

Reset to default
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I inverted color in java before and for each channel in the byte array I would subtract the existing value from 255 a bit like so

ByteArrayOutputStream baos = new ByteArrayOutputStream();
ImageIO.write(img, "jpg", baos);
baos.flush();
byte[] imageBytes = baos.toByteArray();
baos.close();
for(int i = 0; i < bytes.length/3; i++)
{
   //red
   imageBytes[i*4] = (255-imageBytes[i]);
   //green
   imageBytes[(i*4)+1] = (255-imageBytes[(i*4)+1]);
   //Blue
   imageBytes[(i*4)+2] = (255-imageBytes[(i*4)+2]);
}

So for an ARGB I would find which order the Alpha comes in assuming its the first byte in order then your code would read

ByteArrayOutputStream baos = new ByteArrayOutputStream();
ImageIO.write(img, "jpg", baos);
baos.flush();
byte[] imageBytes = baos.toByteArray();
baos.close();
for(int i = 0; i < bytes.length/4; i++)
{
   //alpha
   imageBytes[i*4] = (255-imageBytes[i*4]);
   //red
   //imageBytes[(i*4)+1] = (255-imageBytes[(i*4)+1]);
   //green
   //imageBytes[(i*4)+2] = (255-imageBytes[(i*4)+2]);
   //Blue
   //imageBytes[(i*4)+3] = (255-imageBytes[(i*4)+3]);
}

you would divide bytes.length by 3 for an RGB image and by 4 for an ARGB in the for loop you can then convert to them back to DataBufferByte with

buf.setData(imageBytes);

assuming DataBufferByte works the way I think it does. not hugely familiar with it. I commented the other channels so yo can see what I mean but they aren't needed.

I found this incredibly fast on slow machines as I was able to load and parse an image 512x512 then save it in under a second. I also read somewhere that iterating backwards through an array in java is faster. http://www.mkyong.com/java/reverse-loop-versus-forward-loop-in-performance-java/ I would also note that an ARGB image needs to be png or bmp not jpg like in my example.

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  • \$\begingroup\$ Wouldn't it be for(int i = 0; i < bytes.length; i += 4) instead? \$\endgroup\$ Commented Jan 24, 2014 at 5:08
  • \$\begingroup\$ both for(int i = 0; i < bytes.length; i += 4) and for(int i = 0; i < bytes.length/4; i++) should work fine. In my version you are just shortening the iteration length which is potentially faster \$\endgroup\$ Commented Jan 24, 2014 at 5:12
  • \$\begingroup\$ also geekinterview.com/question_details/42577 apparently i++ is faster than i+=1 thus I would assume i++ is faster than i+=4 \$\endgroup\$ Commented Jan 24, 2014 at 5:37
  • \$\begingroup\$ I forgot you need to turn imageBytes[i] = (255-imageBytes[i]); into imageBytes[i*4] = (255-imageBytes[i*4]); using my iterator. forgive me it's been a while \$\endgroup\$ Commented Jan 24, 2014 at 5:42

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