Agg: Use 32-bit scan line classes

QuLogic · QuLogic · commit 7d73e2a42633 · 2024-10-02T03:12:22.000-04:00
When rendering objects, Agg rasterizes them into scan line objects (an x/y point, horizontal length, and colour), and the renderer class writes those to the pixels in the final buffer. Though we have determined that Agg buffers cannot be larger than 2**16, the scan line classes that we use contain 16-bit _signed_ integers internally, cutting off positive values at half the maximum. Since the renderer uses 32-bit integers, this can cause odd behaviour where any horizontal span that _starts_ before 2**15 (such as a horizontal spine) is rendered correctly even if it cross that point, but those that start after (such as a vertical spine or any portion of an angled line) end up clipped. For example, see how the spines and lines break in #28893. A similar problem occurs for resampled images, which also uses Agg scanlines internally. See the breakage in #26368 for an example. The example in that issue also contains horizontal spines that are wider than 2**15, which also exhibit strange behaviour. By moving to 32-bit scan lines, positions and lengths of the lines will no longer be clipped, and this fixes rendering on very large figures. Fixes #23826 Fixes #26368 Fixes #28893
diff --git a/src/_backend_agg.h b/src/_backend_agg.h
@@ -117,10 +117,10 @@ class RendererAgg
     typedef agg::renderer_scanline_bin_solid<renderer_base> renderer_bin;
     typedef agg::rasterizer_scanline_aa<agg::rasterizer_sl_clip_dbl> rasterizer;
 
-    typedef agg::scanline_p8 scanline_p8;
-    typedef agg::scanline_bin scanline_bin;
+    typedef agg::scanline32_p8 scanline_p8;
+    typedef agg::scanline32_bin scanline_bin;
     typedef agg::amask_no_clip_gray8 alpha_mask_type;
-    typedef agg::scanline_u8_am<alpha_mask_type> scanline_am;
+    typedef agg::scanline32_u8_am<alpha_mask_type> scanline_am;
 
     typedef agg::renderer_base<agg::pixfmt_gray8> renderer_base_alpha_mask_type;
     typedef agg::renderer_scanline_aa_solid<renderer_base_alpha_mask_type> renderer_alpha_mask_type;
diff --git a/src/_image_resample.h b/src/_image_resample.h
@@ -712,6 +712,7 @@ void resample(
 
     using renderer_t = agg::renderer_base<output_pixfmt_t>;
     using rasterizer_t = agg::rasterizer_scanline_aa<agg::rasterizer_sl_clip_dbl>;
+    using scanline_t = agg::scanline32_u8;
 
     using reflect_t = agg::wrap_mode_reflect;
     using image_accessor_t = agg::image_accessor_wrap<input_pixfmt_t, reflect_t, reflect_t>;
@@ -739,7 +740,7 @@ void resample(
 
     span_alloc_t span_alloc;
     rasterizer_t rasterizer;
-    agg::scanline_u8 scanline;
+    scanline_t scanline;
 
     span_conv_alpha_t conv_alpha(params.alpha);
 
