The JHDR chunk introduces the embedded JNG, defines its dimensions, and declares it to have an alpha channel. It is followed by an sRGB PNG chunk that indicates the image is in the International Color Consortium's standard RGB color space; decoders without access to a color management system should instead use the predetermined gamma and chromaticity values that approximate the sRGB color space (see Table 10-3).

The color-space chunk is followed by the IDAT chunks that define the image's alpha channel and the JDAT chunks that define its main (foreground) image. We've included a two-way interleave here in order to allow some possibility of progressive display, but in general one would want to interleave the IDATs and JDATs after perhaps every 16 or 32 rows--16 is a special number for JPEG decoders, and 16 or 32 rows is usually a reasonable amount to display at a time unless the image is quite skinny. On the other hand, keep in mind that each interleave (interleaf) adds an extra 24 bytes of IDAT/JDAT wrapper code; this overhead should be balanced against the desired smoothness of the progressive output.

Note that we've included an IDAT first. This may be a good idea since the decoder often will be able to start displaying the image before all of the JDAT arrives, and we've assumed that the alpha channel is simple enough that the PNG data compressed extremely well (i.e., the IDAT is smaller than the JDAT of the same region). If the reverse is true, the JDAT should come first so that the image can be displayed as each line of alpha channel arrives and is decoded.

Also note that, although I've referred to ``progressive'' display here, I am not necessarily referring either to progressive JPEG or to interlaced PNG. In fact, MNG prohibits interlaced PNG alpha channels in JNG streams, and progressive JPEG may not mix well even with noninterlaced alpha channels, depending on how the application is written. The reason is that the final value of any given pixel will not be known until the JPEG is almost completely transmitted, and ``approximate rendering'' of partially transparent pixels (that is, rendering before the final values are known) requires that the unmodified background image remain available until the end, so that the approximated pixels can be recomputed during the final pass. Of course, a sophisticated decoder could display such an image progressively anyway, but it would incur a substantially greater memory and computational overhead than would be necessary when displaying a nonprogressive JPEG interleaved with an alpha channel. Instead, most decoders are likely to wait for sections of the image (e.g., the first 32 rows) to be competely transmitted before displaying anything. If progressive JPEG data is interleaved with the alpha channel, then such decoders will end up waiting for practically the entire image to be transmitted before even starting to render, which defeats the purpose of both interleaved JNG and progressive JPEG.