diff --git a/contrib/libtests/pngimage.c b/contrib/libtests/pngimage.c
index be176b2bc..1ee8568e5 100644
--- a/contrib/libtests/pngimage.c
+++ b/contrib/libtests/pngimage.c
@@ -1105,6 +1105,7 @@ compare_read(struct display *dp, int applied_transforms)
       {
          unsigned long y;
          int bpp;   /* bits-per-pixel then bytes-per-pixel */
+         int ppb;   /* pixels per byte, or 1 for multi-byte pixels */
          /* components are up to 8 bytes in size */
          png_byte sig_bits[8];
          png_color_8p sBIT;
@@ -1187,16 +1188,19 @@ compare_read(struct display *dp, int applied_transforms)
                   sig_bits[2*b+1] = (png_byte)sig;
                   sig_bits[2*b+0] = (png_byte)(sig >> 8); /* big-endian */
                }
+               ppb = 1;
                break;
 
             case 8: /* One byte per component */
                for (b=0; b*8 < bpp; ++b)
                   sig_bits[b] = (png_byte)(0xff00 >> sig_bits[b]);
+               ppb = 1;
                break;
 
             case 1: /* allowed, but dumb */
                /* Value is 1 */
                sig_bits[0] = 0xff;
+               ppb = 8;
                break;
 
             case 2: /* Replicate 4 times */
@@ -1205,6 +1209,7 @@ compare_read(struct display *dp, int applied_transforms)
                b |= b << 2;
                b |= b << 4;
                sig_bits[0] = (png_byte)b;
+               ppb = 4;
                break;
 
             case 4: /* Replicate twice */
@@ -1212,10 +1217,12 @@ compare_read(struct display *dp, int applied_transforms)
                b = 0xf & ((0xf << 4) >> sig_bits[0]);
                b |= b << 4;
                sig_bits[0] = (png_byte)b;
+               ppb = 2;
                break;
 
             default:
                display_log(dp, LIBPNG_BUG, "invalid bit depth %d", bit_depth);
+               ppb = 1;
                break;
          }
 
@@ -1240,7 +1247,7 @@ compare_read(struct display *dp, int applied_transforms)
             png_bytep orig = dp->original_rows[y];
             unsigned long x;
 
-            for (x=0; x<(width-(mask!=0)); ++x)
+            for (x=0; x<(width-(mask!=0)); x += ppb)
             {
                int b;