premultiply alpha

diff --git a/lib/pdf/GFXOutputDev.cc b/lib/pdf/GFXOutputDev.cc
index 210f88d..81cb642 100644 (file)
--- a/lib/pdf/GFXOutputDev.cc
+++ b/lib/pdf/GFXOutputDev.cc
@@ -2717,6 +2717,10 @@ void GFXOutputDev::setSoftMask(GfxState *state, double *bbox, GBool alpha, Funct
     states[statepos].softmask = 1;
 }
 
+static inline Guchar div255(int x) {
+  return (Guchar)((x + (x >> 8) + 0x80) >> 8);
+}
+
 void GFXOutputDev::clearSoftMask(GfxState *state)
 {
     if(!states[statepos].softmask)
@@ -2779,14 +2783,12 @@ void GFXOutputDev::clearSoftMask(GfxState *state)
        gfxcolor_t* l1 = &maskimg->data[maskimg->width*y];
        gfxcolor_t* l2 = &belowimg->data[belowimg->width*y];
        for(x=0;x<width;x++) {
-           l1->a = 255;
            l2->a = (77*l1->r + 151*l1->g + 28*l1->b) >> 8;
 
-           /* premultiply alpha... do we need this? (depends on output device)
-           l2->r = (l2->a*l2->r) >> 8;
-           l2->g = (l2->a*l2->g) >> 8;
-           l2->b = (l2->a*l2->b) >> 8;
-           */
+           /* premultiply alpha */
+           l2->r = div255(l2->a*l2->r);
+           l2->g = div255(l2->a*l2->g);
+           l2->b = div255(l2->a*l2->b);
 
            l1++;
            l2++;