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swscale: fix overflows in output of RGB48 pixels.

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For certain types of filters where the intermediate sum of coefficients
can go above the fixed-point equivalent of 1.0 in the middle of a filter,
the sum of a 31-bit calculation can overflow in both directions and can
thus not be represented in a 32-bit signed or unsigned integer. To work
around this, we subtract 0x40000000 from a signed integer base, so that
we're halfway signed/unsigned, which makes it fit even if it overflows.
After the filter finishes, we add the scaled bias back after a shift.

We use the same trick for 16-bit bpc YUV output routines.

Signed-off-by: Mans Rullgard <mans@mansr.com>
This commit is contained in:
Ronald S. Bultje 2011-12-16 00:03:54 +00:00 committed by Mans Rullgard
parent 5d8122db5c
commit be1bafc303
1 changed files with 4 additions and 2 deletions
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6
libswscale/swscale.c
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@ -754,8 +754,8 @@ yuv2rgb48_X_c_template(SwsContext *c, const int16_t *lumFilter,
for (i = 0; i < (dstW >> 1); i++) {
int j;
int Y1 = 0;
int Y2 = 0;
int Y1 = -0x40000000;
int Y2 = -0x40000000;
int U = -128 << 23; // 19
int V = -128 << 23;
int R, G, B;
@ -771,7 +771,9 @@ yuv2rgb48_X_c_template(SwsContext *c, const int16_t *lumFilter,
// 8bit: 12+15=27; 16-bit: 12+19=31
Y1 >>= 14; // 10
Y1 += 0x10000;
Y2 >>= 14;
Y2 += 0x10000;
U >>= 14;
V >>= 14;