"File formats: PNG" configuration dialog

This dialog allows to specify parameters of PNG file / stream format.

"Color space" group box

This group box allows you to specify a color space used as an internal format in the PNG 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	Colors in the color RGB image
1	2	2	not applied
2	4	4	not applied
4	16	16	not applied
8	256	256	16777216
16	65536	not applied	281474976710656

Click a radio button in this group box to select a required bit depth.

"Interlaced" check box

This check box allows you to specify whether internally PNG image will be saved as progressive or interlaced.

"Compression" level edit and spin boxes

These control boxes allow you to specify compression level of the PNG image.

The higher a level of compression, the smaller a size of the compressed file. But compression to the higher degree requires more time.

Minimum level of compression is 0, maximum is 9.

At the modern PC compression of a single standard resolution image (640 x 480) is fast at all levels of compression. Therefore maximum compression level 9 is adequate in the majority of occasional situations.

Compression time becomes noticeable when size of image is much bigger than standard resolution, or when multiple images have to be compressed in real time. For example, when PNG format is used for saving a video stream to the hard disk. In that case a lower level of compression may be useful.

"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.

"Remarks

PNG file format uses lossless compression algorithms. It means that color information is not lost in the PNG image during a compression itself, and a quality of an image converted into the PNG format is same as it was the original uncompressed image.

There are two major exceptions from this rule. PNG 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 PNG 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.

For examlpe, color information is lost when original color image is saved as a gray PNG image; or when original image with 8 bit color depth is saved as a 4 bit PNG image.