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  Isolating PNG chunks  (Read 971 times)
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Offline darcone

Junior Devvie




Size matters


« Posted 2004-04-01 17:14:17 »

Hey... I am working on software to join many PNG images with the same header into one file to save space and I´ve been able to after some carefull reading of the PNG specification to get the following:

F:\Development\Java\PNGPrime\bin>java com.qiui.apps.PNGPrime.PNGInfo tank.png
8240 80 78 71 13 10 26 10

A: 0 B: 0 C: 0 D: 13 => 13
Code: IHDR
CRC: 221

A: 0 B: 0 C: 0 D: 4 => 4
Code: gAMA
CRC: 489

A: 0 B: 0 C: 0 D: 25 => 25
Code: tEXt
CRC: 60

A: 0 B: 0 C: 0 D: 48 => 48
Code: PLTE
CRC: 180

A: 0 B: 0 C: 0 D: 1 => 1
Code: tRNS
CRC: 102

A: 0 B: 0 C: 0 D: 25 => 25
Code: IDAT
CRC: 502

A: 56 B: 86 C: 46 D: 24 => 945171992
Code: δ╪rq
Exception in thread "main" java.lang.OutOfMemoryError
_

As you see, something goes wrong after the IDAT chunk, I suspect something is wrong with the length field (but the image displays successfully). This is how I calculate the length:

byte a = (byte)fr.read(), b = (byte)fr.read(), c = (byte)fr.read(), d = (byte)fr.read();
int length = (a << 24) | (b << 16) | (c << Cool | d;

Any ideas guys?

Offline darcone

Junior Devvie




Size matters


« Reply #1 - Posted 2004-04-02 07:06:55 »

Heh, the reason why this did not work is that Adobe Imageready saves down PNG with (probably intentional) errors... Errors that are easy to recover from (thats probably why the image still works everywhere) but gives people like me a hard time.
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