avcodec/mjpegdec: Simplify masking in ljpeg_decode_yuv_scan()

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/mjpegdec.c

@@ -886,12 +886,13 @@ static int ljpeg_decode_rgb_scan(MJpegDecodeContext *s, int nb_components, int p
 static int ljpeg_decode_yuv_scan(MJpegDecodeContext *s, int predictor,
                                  int point_transform, int nb_components)
 {
-    int i, mb_x, mb_y;
+    int i, mb_x, mb_y, mask;
     int bits= (s->bits+7)&~7;
     int resync_mb_y = 0;
     int resync_mb_x = 0;
 
     point_transform += bits - s->bits;
+    mask = ((1 << s->bits) - 1) << point_transform;
 
     av_assert0(nb_components>=1 && nb_components<=4);
 
@@ -944,7 +945,7 @@ static int ljpeg_decode_yuv_scan(MJpegDecodeContext *s, int predictor,
 
                         if (s->interlaced && s->bottom_field)
                             ptr += linesize >> 1;
-                        pred &= (-1)<<(8 - s->bits + point_transform);
+                        pred &= mask;
                         *ptr= pred + (dc << point_transform);
                         }else{
                             ptr16 = (uint16_t*)(s->picture.data[c] + 2*(linesize * (v * mb_y + y)) + 2*(h * mb_x + x)); //FIXME optimize this crap
@@ -964,7 +965,7 @@ static int ljpeg_decode_yuv_scan(MJpegDecodeContext *s, int predictor,
 
                             if (s->interlaced && s->bottom_field)
                                 ptr16 += linesize >> 1;
-                            pred &= (-1)<<(16 - s->bits + point_transform);
+                            pred &= mask;
                             *ptr16= pred + (dc << point_transform);
                         }
                         if (++x == h) {
@@ -1000,13 +1001,13 @@ static int ljpeg_decode_yuv_scan(MJpegDecodeContext *s, int predictor,
                               (h * mb_x + x); //FIXME optimize this crap
                             PREDICT(pred, ptr[-linesize-1], ptr[-linesize], ptr[-1], predictor);
 
-                            pred &= (-1)<<(8 - s->bits + point_transform);
+                            pred &= mask;
                             *ptr = pred + (dc << point_transform);
                         }else{
                             ptr16 = (uint16_t*)(s->picture.data[c] + 2*(linesize * (v * mb_y + y)) + 2*(h * mb_x + x)); //FIXME optimize this crap
                             PREDICT(pred, ptr16[-linesize-1], ptr16[-linesize], ptr16[-1], predictor);
 
-                            pred &= (-1)<<(16 - s->bits + point_transform);
+                            pred &= mask;
                             *ptr16= pred + (dc << point_transform);
                         }