Allows you to configure settings for saving a PNG file. All applications which can handle PNG files should be able to read any PNG saved from GIMP. However you can make decisions here about which information about your image should be preserved in the file, and how hard GIMP should try to keep the file size small.
PNG File Settings
The Adam7 interlacing option stores image data in an order which allows browsers or viewers to show progressively more detail as they load the file, at the cost of increased file size.
This adds a bKGD chunk to your PNG file which advises viewer software to choose a background color like your currently selected GIMP background color. Adding this chunk increases file size slightly. Netscape Navigator will not display a png transparently if the file has been saved with this option. It will instead use the chosen background color behind the image.
This adds a gAMA chunk to your PNG file which reflects the current Gamma factor for your display (set globally for GIMP). Viewers with a different display can compensate to ensure the image doesn't look too dark or too bright. Adding this chunk increases file size slightly.
If you are saving a single layer which has been shifted (offset) from the top-left of the image, this will add a oFFs chunk to your PNG file to preserve this offset factor in other viewers. Adding this chunk if it's needed increases file size slightly.
This adds a pHYs chunk to your PNG which records the physical resolution of the image. This is needed by GIMP and other viewers for printing or actual size previews. Adding this chunk increases file size slightly.
This adds a tIME chunk to your PNG which records the exact date and time when the image was last saved. This will be preserved and can be viewed in other applications. Adding this chunk increases file size slightly.
This controls the amount of compression used to reduce the file size. Larger numbers will usually reduce file size but will make saving slower. The default (6) is often an acceptable compromise, but see below.