I bluffed myself in the corner again ...

Question

Greetings,
My neighbour's son is programming in Java and I like to see him do that; last night he asked me a question about generating PNG or GIF files or whatever. I mumbled something about the Robot class an ImageIO. This morning he came to me and said that he had implemented (some rudiments of) the GIMP, i.e. he could take a snapshot of some part of the screen and save that in a file with one of those types. So far so good but he wanted to add transparency so I mumbled something about manipulating a BufferedImage, translating a colour to a completely transparent colour on the fly. This afternoon he came back to me saying he couldn't do it; I said I would have a look at it.
I can't do it either! That darn Robot class gives me a BufferedImage of type 1, i.e. of type TYPE_INT_RGB and there is no such thing as an alpha channel in that image. I want to have a type 2 BufferedImage so that I can set any pixel value of 0xAARRGGBB to, say, 0x00000000 i.e. make that colour a transparent colour in the image and save the image in a PNG file. Of course I got lost again in all the Swing API documentation (as always) and still haven't found an acceptible solution yet; another afternoon down the drain.
Can anybody help me out?
kind regards,
Jos

Answer 1

I'll be a little more specific with my answer.
When saving a 24-bit colored image, the png specification does allow for you to specify exactly one transparent color in the metadata. It's callled the "tRNS chunk." You would have to be a little familiar with the javax.imageio.metadata package in order to tell ImageIO that you want to include this tRNS chunk of metadata.
While the alternative I suggested,
You would have to create a new image, one with an alpha channel, and copy/draw the screenshot one onto the alpha channel one. Then you manipulate the alpha channel one to your heart's content.is obvious, it will result in a 32-bit colored png file (a larger file size). But at least it's simple.
There is also a bug in the PNGImageReader when reading a 24-bit colored image with a tRNS chunk. Because there is no way to create a BufferedImage that allows full color, but exactly one transparent color, the tRNS chunk is ignored and the transparent color is lost when reading the png file.