"File formats: GIF" configuration dialog
This dialog allows to specify parameters of GIF file / stream format.
"Color space" group box
This group box allows to specify a color space used as an internal format in the GIF image.Click a radio button in this group box to select a required color space.
"Bit depth" group box
This group box allows you to specify a bit depth of each color component.Table below shows how many colors there are in the images with the different bit detpth:
| Bit depth | Colors in the grayscale image | Colors in the color palette image |
| 1 | 2 | 2 |
| 2 | 4 | 4 |
| 4 | 16 | 16 |
| 8 | 256 | 256 |
Click a radio button in this group box to select a required bit depth.
"Palette" group box
This group box allows you to specify parameters of algorithms used to build a palette when "Color palette" radio button in the "Color space" group box is checked.if "Standard halftone" radio button is checked then palletized images are made with the standard for Windows operating system halftone palette. This method is fast but colors of that palette are not optimal for concrete images.
If "Optimal adaptive octree" radio button is checked then palletized images are made with the adaptive octree palette (Gervautz-Purgathofer octree algorithm). This method is slower but colors of that palette are adaptively selected for every concrete image. As a result quality of a final image is usually much higher.
"Significant color bits" edit and spin boxes allow setting a number of color bits used in the octree algorithm. Usually from 4 to 6 bits are enough to make a good quality palette. Increasing number of bits to 7 and 8 does not usually improve noticeably quality of palette. Best palette is made when the maximum number of significant bits, 8, But using a bigger number of bits makes calculations slower.
"Interlaced" check box
This check box allows you to specify whether internally PNG image will be save as progressive or interlaced.
"Remarks
GIF file format uses a lossless compression algorithm. It means that color information is not lost in the GIF image during compression itself, and a quality of an image converted into the GIF format may be the same as it was the original uncompressed image.There are two major exceptions from this rule. GIF image internally may be presented in the different color spaces and with the different color bit depth. As a result some color information may be lost if during GIF compression:
- Color space of the original image is converted into another color space with a smaller number of color components.
- Bit depth of the original image is reduced.
GIF color images contain no more than 256 colors. Therefore color information is lost when 24 bit RGB image is saved as a GIF. Also color information is lost when original color image is saved as a gray GIF image; or when original image with 8 bit color depth is saved as a 4 bit GIF image.