One way to reduce the size of a J2ME application is through better compression of the PNG images you are using.
I currently use the excellent Paint.Net to create my images. Unfortunately, it doesn’t compress PNG output very effectively. It is particularly poor with monochrome images. Ideally, these would be saved in 2-bit color mode, but the best Paint.Net can do is 8-bit color. Surprisingly, this limitation is not unique to Paint.Net: numerous other tools have the same limitation. For example, I have created a monochrome image (containing a bitmapped font) and this was saved by Paint.Net using 8-bit color. It weighed in at 2352 bytes.
One of Paint.Net’s best features is its support for a wide range of plugins. One of these is able to host a PNG compressor called OptiPng. This is able to save 2-bit color and reduce the file down to just 928 bytes. A huge improvement.
Better still, a stand-alone little utility called PngGantlet uses the incredible PNGOOUT compressor to achieve even better results. It was able to shrink this image down to a mere 806 bytes: that’s just 34% of the size of the original image, with no loss of quality.
This is all the more impressive when you consider that the same image as a BMP file comes in at a massive 14,902 bytes (806 = 5.4% of 14902).
Here’s a summary of my results:
| File | Size (bytes) | ~% |
|---|---|---|
| BMP | 14,902 | 633% |
| Original PNG Image | 2352 | 100% |
| OptiPNG | 928 | 39% |
| PngGantlet | 806 | 34% |
One caveat – these figures don’t take into account any additional compression undertaken when converting the final application into a .jar file. I assume this compression will be less effective on the final 806-byte file than the original 2353-byte file.
Nevertheless, I don’t doubt that applying the compressor to all my images will result in a significantly smaller application.