png IDAT数据块还原问题

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png由于fireworks生成的数再进一步处理，还原出来的数据还要计算下

这两天查遍了互联网，和无数次生成数据手工计算还原，希望找到个规律

从一篇提示想到手工猜测，我用php写了一个模板，枚举了很多种算法。直接显示结果

如果能还原出来跟fireworks看到的值一样说明民成功的。

这里有一个仁兄提问，他说是位运算

我后来发现是加减运算

http://tieba.baidu.com/f?kz=859640399

01 就是后面一个RGBA加上前边一个RGBA

02 就是下面原始数据加上上面解码出来的RGBA

04 在下面有说明

03 是前边一个字节加上上边对应一个字节然后除2 然后加上本字节比如(前R+上R)/2+R(本字节R)

这个适合真彩情况，window画图生成的png并没有编码,直接放RRGGBBAA

我推理了一天才发现03这种情况所以如果png IDAT 遇到真彩解码或编码问题可以多试下

行头为04数据格式

00 C B04 A X

Type byteFilter namePredicted value0NoneZero (so that the raw byte value passes through unaltered)1SubByte A (to the left)2UpByte B (above)3AverageMean of bytes A and B, rounded down4PaethA, B, or C, whichever is closest to p = A + B − C
Table 1. Predictor states used for differential encoding in the PNG image format.
The Paeth filter computes a simple linear function of the three neighboring pixels (a, b, c), then
chooses as predictor the neighboring pixel closest to the computed value as defined by the
following pseudo-code:
// Input: color values a,b, and c as illustrated in Figure 3
// a = left, b = above, c = upper left
// Output: a paeth-prediction for a,b, and c

04方式求出X的伪代码
paeth_predict(a,b,c)
p := a+b-c
pa := abs(p-a)
pb := abs(p-b)
pc := abs(p-c)
IF (pa<=pb AND pa<=pc) p := a
ELSE IF (pb <= pc) p := b
ELSE p := c
paeth_predict := p
Compression of a pixel value x dependent on its neighbors a,b, and c works by calculating
compressed(x) = x - paeth_predict(a,b,c)
and decompression works by reversing the formula to
uncompressed(x) = compressed(x) + paeth_predict(a,b,c).

http://en.wikipedia.org/wiki/Portable_Network_Graphics

http://ngs.ics.uci.edu/teaching/winter2011/Multimedia/textbook/MM_Chapter_lossycomp_100220.pdf

具体看下面这里

http://www.w3.org/TR/PNG/#9Filters