Catch slow case in PDF tiling.

Sometimes a PDF file can use just a very small amount of a large tile;
rendering the whole tile is a waste of time. Previous attempts to catch
this case have worked for most cases, but can be defeated when the
area needing rendering crosses a tile edge.

Here we improve the formulation to catch exactly what we want.

This implementation was driven by the example file in Sumatra PDFs
bug tracker, for issue 2248, though the actual code change is
different. Thanks to zeniko, nonetheless for this!

1 files changed, 21 insertions, 9 deletions

diff --git a/pdf/pdf_interpret.c b/pdf/pdf_interpret.c
index a08fbb8f..cebc853d 100644
--- a/pdf/pdf_interpret.c
+++ b/pdf/pdf_interpret.c
@@ -1325,6 +1325,7 @@ pdf_show_pattern(pdf_csi *csi, pdf_pattern *pat, const fz_rect *area, int what)
 	fz_matrix ptm, invptm;
 	fz_matrix oldtopctm;
 	int x0, y0, x1, y1;
+	float fx0, fy0, fx1, fy1;
 	int oldtop;
 	fz_rect local_area;
 
@@ -1370,20 +1371,20 @@ pdf_show_pattern(pdf_csi *csi, pdf_pattern *pat, const fz_rect *area, int what)
 	local_area = *area;
 	fz_transform_rect(&local_area, &invptm);
 
-	/* When calculating the number of tiles required, we adjust by a small
-	 * amount to allow for rounding errors. By choosing this amount to be
-	 * smaller than 1/256, we guarantee we won't cause problems that will
-	 * be visible even under our most extreme antialiasing. */
-	x0 = floorf((local_area.x0 - pat->bbox.x0) / pat->xstep + 0.001);
-	y0 = floorf((local_area.y0 - pat->bbox.y0) / pat->ystep + 0.001);
-	x1 = ceilf((local_area.x1 - pat->bbox.x0) / pat->xstep - 0.001);
-	y1 = ceilf((local_area.y1 - pat->bbox.y0) / pat->ystep - 0.001);
+	fx0 = (local_area.x0 - pat->bbox.x0) / pat->xstep;
+	fy0 = (local_area.y0 - pat->bbox.y0) / pat->ystep;
+	fx1 = (local_area.x1 - pat->bbox.x0) / pat->xstep;
+	fy1 = (local_area.y1 - pat->bbox.y0) / pat->ystep;
 
 	oldtopctm = csi->top_ctm;
 	oldtop = csi->gtop;
 
 #ifdef TILE
-	if ((x1 - x0) * (y1 - y0) > 1)
+	/* We have tried various formulations in the past, but this one is
+	 * best we've found; only use it as a tile if a whole repeat is
+	 * required in at least one direction. Note, that this allows for
+	 * 'sections' of 4 tiles to be show, but all non-overlapping. */
+	if (fx1-fx0 > 1 || fy1-fy0 > 1)
 #else
 	if (0)
 #endif
@@ -1401,6 +1402,17 @@ pdf_show_pattern(pdf_csi *csi, pdf_pattern *pat, const fz_rect *area, int what)
 	else
 	{
 		int x, y;
+
+		/* When calculating the number of tiles required, we adjust by
+		 * a small amount to allow for rounding errors. By choosing
+		 * this amount to be smaller than 1/256, we guarantee we won't
+		 * cause problems that will be visible even under our most
+		 * extreme antialiasing. */
+		x0 = floorf(fx0 + 0.001);
+		y0 = floorf(fy0 + 0.001);
+		x1 = ceilf(fx1 - 0.001);
+		y1 = ceilf(fy1 - 0.001);
+
 		for (y = y0; y < y1; y++)
 		{
 			for (x = x0; x < x1; x++)