Adam7 algorithm

Adam7 is an algorithm for progressive image structure in the transfer of image data over a slow connection such as the Internet. The goal is to show the user instead of a picture that builds up slowly from top to bottom, a rough version of the image that gradually refines. Adam7 is named after its inventor, Adam M. Costello and is used among other things in the PNG image format.

Method

The overall picture is built up in a total of seven runs. For this purpose, it is first necessary to divide the original image into blocks of 8 × 8 pixels. Each pixel has a certain value 1 to 7 in the image file are sequentially stored the seven runs. Each run consists of the pixels with the corresponding value. When reading the file at the corresponding runs are transmitted consecutively.

The number of pixels increases with each pass or remains the same. The weights are defined as follows:

It is customary to draw for the existing pixels on the remaining, not yet transmitted image block rectangles, giving the impression of a gradual " refinement " is created.

Pros and Cons

Has Adam7 from that used in GIF scheme in which only one full lines are transmitted, the advantage that the first pass affects less pixels and thus an output image can be presented as soon eight times. In addition, the refinement of columns helps in addition to lines to make faster text legible.

However Adam7 is to implement more complicated. In particular, care must be taken that even images that are less than a block to be evaluated correctly. Compressed image files that use Adam7, are also usually slightly larger than normal image files, as the natural order of the image data is not retained. Therefore Adam7 should be stored only in larger images that are posted on the Internet or on a slow storage medium may be used.