搞定了
State Name Predicted value
0 None Zero (so that the raw value passes through unaltered)
1 Sub Value of a (to the left)
2 Up Value of b (above)
3 Average Mean of bytes a and b, rounded down
4 Paeth a, 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
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).
