On Sun, Apr 26, 2009 at 8:01 AM, yahvuu <yahvuu@xxxxxxxxx> wrote: > peter sikking schrieb: >> I like the innovative nature of the idea. > And i'm not aware of a raster file format which features that concept. I just want to point out that PNG supports a background color (and the GIMP plugin to save PNG offers an option to save the current brush background color as the image background color), and being the only format to do so we should probably consider its specified functions and suggested behavior as potential starting points for GIMP's handling of such. 4.2.4.1. bKGD Background color The bKGD chunk specifies a default background color to present the image against. Note that viewers are not bound to honor this chunk; a viewer can choose to use a different background. For color type 3 (indexed color), the bKGD chunk contains: Palette index: 1 byte The value is the palette index of the color to be used as background. For color types 0 and 4 (grayscale, with or without alpha), bKGD contains: Gray: 2 bytes, range 0 .. (2^bitdepth)-1 (If the image bit depth is less than 16, the least significant bits are used and the others are 0.) The value is the gray level to be used as background. For color types 2 and 6 (truecolor, with or without alpha), bKGD contains: Red: 2 bytes, range 0 .. (2^bitdepth)-1 Green: 2 bytes, range 0 .. (2^bitdepth)-1 Blue: 2 bytes, range 0 .. (2^bitdepth)-1 (If the image bit depth is less than 16, the least significant bits are used and the others are 0.) This is the RGB color to be used as background. When present, the bKGD chunk must precede the first IDAT chunk, and must follow the PLTE chunk, if any. See Recommendations for Decoders: Background color. ========================================= 10.7. Background color The background color given by bKGD will typically be used to fill unused screen space around the image, as well as any transparent pixels within the image. (Thus, bKGD is valid and useful even when the image does not use transparency.) If no bKGD chunk is present, the viewer will need to make its own decision about a suitable background color. Viewers that have a specific background against which to present the image (such as Web browsers) should ignore the bKGD chunk, in effect overriding bKGD with their preferred background color or background image. The background color given by bKGD is not to be considered transparent, even if it happens to match the color given by tRNS (or, in the case of an indexed-color image, refers to a palette index that is marked as transparent by tRNS). Otherwise one would have to imagine something "behind the background" to composite against. The background color is either used as background or ignored; it is not an intermediate layer between the PNG image and some other background. Indeed, it will be common that bKGD and tRNS specify the same color, since then a decoder that does not implement transparency processing will give the intended display, at least when no partially-transparent pixels are present. _______________________________________________ Gimp-developer mailing list Gimp-developer@xxxxxxxxxxxxxxxxxxxxxx https://lists.XCF.Berkeley.EDU/mailman/listinfo/gimp-developer
