In your bitmap graphics editor, set your fuzzy selection ("magic wand") tool so that it only will select pixels of exactly the same color. Suppose you have a map for an island that was inadvertently saved as JPEG. Making an image that was incorrectly saved as JPEG fit for saving as PNG
To remedy this, you can use the method described below to make sure that there are no more colors in the image than you actually need. PNG's compression method needlessly preserves these differences, and the result of converting a JPEG to PNG may not yield the reduction in file size that you hoped for. Once an image such as a map has been saved as JPEG inadvertently, it often contains neighboring colors that look the same to the human eye, but which are in fact different.
To further reduce file size, you can use one of the optimization programs mentioned at the Wikipedia graphics tutorial page. You can try using pngquant on the 24-bit result this produces indexed mode images usually with few artifacts. Using the posterize option of your graphics editor with a high setting (like 40) may further reduce filesize and eliminate more artifacts without visibly reducing image quality. Instead, just eliminate JPEG artifacts as much as possible and convert to 24-bit PNG. Some tools may let you count the actual number of different colors in a file.įor images containing smooth gradients or a lot of detail, avoid moving to indexed mode, as this can lose information and cause banding artifacts. Experiment until you get it a result you like. You will probably want to turn this option off. They may also ask if you want to use something called "error diffusion" or "dithering". How many colors you choose will depend upon the image-it must be fewer than 256, but often 16 or even fewer will work well. Most raster graphics editors will let you convert from true-color mode (also called "full color" or "RGB"), and let you decide the number of colors you want in the resulting indexed image. You should convert such PNGs to "indexed" or "palette-based" mode before you upload them. If you merely convert a JPEG to a PNG, your graphics program will likely produce one of these true-color PNGs, and make a large file. However, when you save an image as a true color PNG, you tend to get overly large files, since its compression is not optimized for such images. You can store images as PNG files while retaining more colors than the human eye can possibly distinguish, and can represent more colors and finer details than even JPEG, but only when saved in PNG's "true color" mode.
In addition, though, the PNG format is very versatile. Its compression is optimized for images with large areas of identical color with sharp edges. PNG, however, uses lossless compression, which means that it saves every pixel exactly as it was drawn, which is ideal for iconic images such as graphs, flags, maps, and cartoons and for images likely to undergo further editing. JPEG's compression method takes advantage of the fact that most natural scenes have smooth gradients and soft edges, and so is optimized for that. When you need to save an image with thousands of colors or more (as is typical in "photographic" images with fine gradations of shade and tone) and you are concerned about filesize, your best bet is probably to save it as a JPEG file.
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