Dialog "Optimize GIF".

To use this dialog select menu item File / Optimize or click the toolbar button i optim (or use "hot key" F5).

Six checkboxes define conditions of optimization. You can use different combinations and see how they influence size of optimized file.
Remove redundant pixels. For animated pictures there is a frequent situation, when different frames use the same colored pixels. For example, each frame contains the same background or some subimage is used simultaneously in several or all frames. With the help of this operation pixels which are used in several frames are deleted. It allows to reduce the sizes of individual frames and, hence, download time of the whole GIF- file will be decreased. Pixels deleted from a transparent image are present at the previous frames. The result depends on used pictures itself very much.

Notes:

  • sometimes total size of the GIF- file can even be increased! Therefore efficiency of this type of optimization checked up in this manner: you switch it on and off.
  • images which you see in the frames after such operation can be strange for you, but you should know that pixels will be combined with pixels from the previous frames.
For example, you can see the result of optimization of a GIF- file consisting of 5 frames:
antenna   antenna1 antenna2 antenna3 antenna4 antenna5
You can restore images in frames of optimized GIFs with help of menu item File / Restore Frames.

Note: in common case we unable to define exact frame that contained the pixel before optimization. Another problem that two transparent frames may have different 256- colored local palettes, so we have image with more than 256 colors and must make color reduction. Sort color map by usage and remove unused entries. For more images, not being photos, the amount of colors used for reproduction frequently is less 256. So it is possible to remove them without any loss for images.

Note that color palette may contain one of the following number of entries: 2, 4, 8, 16, 32, 64, 128, 256. It means, that after removing unused entries size of color palette may be reduced, or may be not. It depends on the number colors in source palette and number of unused colors in image.

Look at two GIFs - they do not differ, but left image uses a full color palette in 256 colors, and right image has palette consisting only of 32 colors. And also these images have different file sizes.
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