Agg: Use 32-bit scan line classes

QuLogic · QuLogic · commit ff86c1988b1c · 2024-09-27T21:04:25.000-04:00
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
@@ -5,6 +5,9 @@
 
 #define MPL_DISABLE_AGG_GRAY_CLIPPING
 
+#include <limits>
+#include <type_traits>
+
 #include "agg_image_accessors.h"
 #include "agg_path_storage.h"
 #include "agg_pixfmt_gray.h"
@@ -699,8 +702,8 @@ static void get_filter(const resample_params_t &params,
 }
 
 
-template<typename color_type>
-void resample(
+template<typename color_type, bool is_large = false>
+void resample_scanline(
     const void *input, int in_width, int in_height,
     void *output, int out_width, int out_height,
     resample_params_t &params)
@@ -712,6 +715,7 @@ 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 = std::conditional_t<is_large, agg::scanline32_u8, agg::scanline_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 +743,7 @@ 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);
 
@@ -828,4 +832,25 @@ void resample(
     }
 }
 
+template<typename color_type>
+void resample(
+    const void *input, int in_width, int in_height,
+    void *output, int out_width, int out_height,
+    resample_params_t &params)
+{
+    // The smaller scanline classes use int16, so use the larger class for larger output.
+    constexpr int cutoff = std::numeric_limits<agg::int16>::max();
+    if (out_width >= cutoff || out_height >= cutoff) {
+        resample_scanline<color_type, true>(
+            input, in_width, in_height,
+            output, out_width, out_height,
+            params);
+    } else {
+        resample_scanline<color_type, false>(
+            input, in_width, in_height,
+            output, out_width, out_height,
+            params);
+    }
+}
+
 #endif /* MPL_RESAMPLE_H */
