Re: seeking for help on MNG creation

Sven Neumann <[email protected]> wrote:

> I have written this tool using libmng and it creates MNG files that
> should IMO work. Judging from the output of mngtree, the files seem to
> be correct and none of the viewers I tried so far complain. But the
> problem is that nothing is shown except the background image. So I
> guess I must be doing something wrong. I have put a simple MNG file
> (2 frames) online here:
>   http://sven.gimp.org/blinkenlights/heart.mng

In general, I would tend to test (current versions of) libmng over
pngcheck, but be aware that the latter shows an invalid length for
the BACK chunk:

> pngcheck -v blinkenlights-heart.mng
File: blinkenlights-heart.mng (80633 bytes)
  chunk MHDR at offset 0x0000c, length 28
    320 x 240 frame size, 1000 ticks per second, unspecified layer count,
    3 frames, 600-tick play time (1 seconds), valid profile:
      simple MNG features, complex MNG features
  chunk tEXt at offset 0x00034, length 25, keyword: title
  chunk DEFI at offset 0x00059, length 2
    object ID = 1, potentially visible, abstract, x = 0, y = 0
  chunk IHDR at offset 0x00067, length 13
    320 x 240 image, 24-bit RGB, non-interlaced
  chunk IDAT at offset 0x00080, length 74127
    zlib:  deflated, 32K window, maximum compression
  chunk IEND at offset 0x1221b, length 0
  chunk BACK at offset 0x12227, length 9:  incorrect length

That said, it's probably due to a spec change from whatever draft was
current when I added BACK support; I haven't had time to go back and
verify that MNG chunk support is up to date (or to finish adding it...).
It would be simple enough to check whether a 9-byte BACK chunk is valid
according to the spec.

Hopefully one of the other MNG folks can come up with some suggestions
if that's not the problem...

Greg

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