From 8cf479739a15b551f6afbb88e1c76c331e61142e Mon Sep 17 00:00:00 2001
From: Tony Cook <tony@develop-help.com>
Date: Fri, 5 Jan 2018 22:09:46 +1100
Subject: [PATCH] (rt #124001) avoid flooring a second time in matrix transform
 interpolation

The linear interpolation done by matrix_transform_bg() (also called
when rotating images) called floor() on the ordinate both in the
caller and in the interpolation code, which seemed to confuse the compiler
supplied with the gcc supplied with 32-bit strawberry perl 5.26.0.

I suspect a value just barely below an integer was being stored in the
FPU and then rounded up to that integer when converted to a double,
which then caused an off-by-one error in interpolation.
---
 rotate.im | 27 +++++++++++++++------------
 1 file changed, 15 insertions(+), 12 deletions(-)

diff --git a/rotate.im b/rotate.im
index 250d778b..96981be7 100644
--- a/rotate.im
+++ b/rotate.im
@@ -130,7 +130,6 @@ static i_color interp_i_color(i_color before, i_color after, double pos,
   i_color out;
   int ch;
 
-  pos -= floor(pos);
   if (channels == 1 || channels == 3) {
     for (ch = 0; ch < channels; ++ch)
       out.channel[ch] = ((1-pos) * before.channel[ch] + pos * after.channel[ch]) + 0.5;
@@ -170,7 +169,6 @@ static i_fcolor interp_i_fcolor(i_fcolor before, i_fcolor after, double pos,
   i_fcolor out;
   int ch;
 
-  pos -= floor(pos);
   if (channels == 1 || channels == 3) {
     for (ch = 0; ch < channels; ++ch)
       out.channel[ch] = (1-pos) * before.channel[ch] + pos * after.channel[ch];
@@ -260,22 +258,26 @@ i_img *i_matrix_transform_bg(i_img *src, i_img_dim xsize, i_img_dim ysize, const
 	if (fabs(sz) > 0.0000001 
 	    && sx >= -1 && sx < src->xsize
 	    && sy >= -1 && sy < src->ysize) {
-	  i_img_dim bx = floor(sx);
-	  i_img_dim by = floor(sy);
+	  double fsx = floor(sx);
+	  double fsy = floor(sy);
+	  i_img_dim bx = fsx;
+	  i_img_dim by = fsy;
 
 	  ROT_DEBUG(fprintf(stderr, "map " i_DFp " to %g,%g\n", i_DFcp(x, y), sx, sy));
-	  if (sx != bx) {
-	    if (sy != by) {
+	  if (sx != fsx) {
+	    double dx = sx - fsx;
+	    if (sy != fsy) {
 	      IM_COLOR c[2][2]; 
 	      IM_COLOR ci2[2];
+	      double dy = sy - fsy;
 	      ROT_DEBUG(fprintf(stderr, " both non-int\n"));
 	      for (i = 0; i < 2; ++i)
 		for (j = 0; j < 2; ++j)
 		  if (IM_GPIX(src, bx+i, by+j, &c[j][i]))
 		    c[j][i] = back;
 	      for (j = 0; j < 2; ++j)
-		ci2[j] = interp_i_color(c[j][0], c[j][1], sx, src->channels);
-	      vals[x] = interp_i_color(ci2[0], ci2[1], sy, src->channels);
+		ci2[j] = interp_i_color(c[j][0], c[j][1], dx, src->channels);
+	      vals[x] = interp_i_color(ci2[0], ci2[1], dy, src->channels);
 	    }
 	    else {
 	      IM_COLOR ci2[2];
@@ -283,22 +285,23 @@ i_img *i_matrix_transform_bg(i_img *src, i_img_dim xsize, i_img_dim ysize, const
 	      for (i = 0; i < 2; ++i)
 		if (IM_GPIX(src, bx+i, sy, ci2+i))
 		  ci2[i] = back;
-	      vals[x] = interp_i_color(ci2[0], ci2[1], sx, src->channels);
+	      vals[x] = interp_i_color(ci2[0], ci2[1], dx, src->channels);
 	    }
 	  }
 	  else {
-	    if (sy != (i_img_dim)sy) {
+	    if (sy != fsy) {
 	      IM_COLOR ci2[2];
+	      double dy = sy - fsy;
 	      ROT_DEBUG(fprintf(stderr, " x int, y non-int\n"));
 	      for (i = 0; i < 2; ++i)
 		if (IM_GPIX(src, bx, by+i, ci2+i))
 		  ci2[i] = back;
-	      vals[x] = interp_i_color(ci2[0], ci2[1], sy, src->channels);
+	      vals[x] = interp_i_color(ci2[0], ci2[1], dy, src->channels);
 	    }
 	    else {
 	      ROT_DEBUG(fprintf(stderr, " both int\n"));
 	      /* all the world's an integer */
-	      if (IM_GPIX(src, sx, sy, vals+x))
+	      if (IM_GPIX(src, bx, by, vals+x))
 		vals[x] = back;
 	    }
 	  }
-- 
2.30.2