Background Color (bKGD)

• Status:   PNG Specification
• Location:   After PLTE, before first IDAT
• Multiple:   no

In some applications, notably web browsers, there is a natural background surrounding all images, against which images can be composited with transparency information. But standalone image viewers typically have no preferred background color or pattern and usually default to black, which may not be appropriate for some images. PNG therefore supports the concept of a preferred background color that can be used if nothing better is available.

The bKGD chunk is used for this purpose. Just as with the transparency chunk, tRNS the format of bKGD depends on the image type. For palette-based images it contains a single byte, whose value is the palette index of the color to be used for the background. For grayscale images, with or without alpha, the chunk contains a 2-byte, unscaled gray value, just as with tRNS--that is, the maximum allowed value is 2^{bit depth}-1, even though it is stored as a 16-bit integer. And for truecolor images, the background chunk is exactly analogous to the grayscale version except that it contains three 16-bit, unscaled values representing the red, green, and blue components of the background color. There is no requirement in any of the three cases that the background color be present in the actual image data.

Note that colored backgrounds are not supported in grayscale images; while this is certainly a restriction, it appears not to be a particularly serious one, to judge by the lack of public comment to date. Note also that the background color should always be considered fully opaque, even if it happens to match a color marked by the tRNS chunk as partly or fully transparent.