Move rendering buffers pointers computations inside FontRendererBlendGammaSubpixel

The logic is to disentangle the code between renderer.c and FontRenderer
so that this latter has a simple, logical interface.

lib/font_renderer/font_renderer.cpp

@@ -277,6 +277,7 @@ void blend_solid_hspan_rgb_subpixel(agg::rendering_buffer& rbuf, agg::gamma_lut<
     }
 }
 
+#if 0
 // destination implicitly BGRA32. Source implictly single-byte renderer_alpha coverage.
 void FontRendererBlendGamma(FontRenderer *font_renderer, uint8_t *dst, int dst_stride, uint8_t *src, int src_stride, int region_width, int region_height, FontRendererColor color) {
     blender_gamma_type& blender = font_renderer->blender();
@@ -287,16 +288,43 @@ void FontRendererBlendGamma(FontRenderer *font_renderer, uint8_t *dst, int dst_s
         blend_solid_hspan(dst_ren_buf, blender, x, y, region_width, color_a, covers);
     }
 }
+#endif
 
 // destination implicitly BGRA32. Source implictly single-byte renderer_alpha coverage with subpixel scale = 3.
-void FontRendererBlendGammaSubpixel(FontRenderer *font_renderer, uint8_t *dst, int dst_stride, uint8_t *src, int src_stride, int region_width, int region_height, FontRendererColor color) {
-    const int subpixel_scale = 3;
+// FIXME: consider using something like RenColor* instead of uint8_t * for dst.
+void FontRendererBlendGammaSubpixel(FontRenderer *font_renderer, FontRendererClipArea *clip, int x, int y, uint8_t *dst, int dst_width, const FontRendererBitmap *glyphs_bitmap, const GlyphBitmapInfo *glyph, FontRendererColor color) {
     agg::gamma_lut<>& gamma = font_renderer->gamma();
     agg::lcd_distribution_lut& lcd_lut = font_renderer->lcd_distribution_lut();
-    agg::rendering_buffer dst_ren_buf(dst, region_width, region_height, dst_stride);
+    const int subpixel_scale = 3;
+    const int pixel_size = 4; // Pixel size for BGRA32 format.
+
+    x += glyph->xoff;
+    y += glyph->yoff;
+
+    int glyph_x = glyph->x0, glyph_y = glyph->y0;
+    int glyph_width  = glyph->x1 - glyph->x0;
+    int glyph_height = glyph->y1 - glyph->y0;
+
+    int n;
+    if ((n = clip->left - x) > 0) { glyph_width  -= n; glyph_x += n; x += n; }
+    if ((n = clip->top  - y) > 0) { glyph_height -= n; glyph_y += n; y += n; }
+    if ((n = x + glyph_width  - clip->right ) > 0) { glyph_width  -= n; }
+    if ((n = y + glyph_height - clip->bottom) > 0) { glyph_height -= n; }
+
+    if (glyph_width <= 0 || glyph_height <= 0) {
+        return;
+    }
+
+    dst += (x + y * dst_width) * pixel_size;
+    agg::rendering_buffer dst_ren_buf(dst, glyph_width, glyph_height, dst_width * pixel_size);
+
+    uint8_t *src = glyphs_bitmap->pixels + (glyph_x + glyph_y * glyphs_bitmap->width) * subpixel_scale;
+    int src_stride = glyphs_bitmap->width * subpixel_scale;
+
     const agg::rgba8 color_a(color.r, color.g, color.b);
-    for (int x = 0, y = 0; y < region_height; y++) {
+    for (int x = 0, y = 0; y < glyph_height; y++) {
         agg::int8u *covers = src + y * src_stride;
-        blend_solid_hspan_rgb_subpixel(dst_ren_buf, gamma, lcd_lut, x, y, region_width * subpixel_scale, color_a, covers);
+        blend_solid_hspan_rgb_subpixel(dst_ren_buf, gamma, lcd_lut, x, y, glyph_width * subpixel_scale, color_a, covers);
     }
 }
+

lib/font_renderer/font_renderer.h

@@ -7,12 +7,19 @@
 extern "C" {
 #endif
 
-// Mirrors stbtt_bakedchar.
 typedef struct {
   unsigned short x0, y0, x1, y1;
   float xoff, yoff, xadvance;
 } GlyphBitmapInfo;
 
+// Important: when a subpixel scale is used the width below will be the width in logical pixel.
+// As each logical pixel contains 3 subpixels it means that the 'pixels' pointer
+// will hold enough space for '3 * width' uint8_t values.
+typedef struct {
+  uint8_t *pixels;
+  int width, height;
+} FontRendererBitmap;
+
 struct FontRendererImpl;
 typedef struct FontRendererImpl FontRenderer;
 
@@ -26,6 +33,10 @@ typedef struct {
     uint8_t r, g, b;
 } FontRendererColor;
 
+typedef struct {
+  int left, top, right, bottom;
+} FontRendererClipArea;
+
 FontRenderer *  FontRendererNew(unsigned int flags, float gamma);
 
 void            FontRendererFree(FontRenderer *);
@@ -39,17 +50,20 @@ int             FontRendererBakeFontBitmap(FontRenderer *, int font_height,
                     int first_char, int num_chars, GlyphBitmapInfo *glyph_info,
                     int subpixel_scale);
 
+#if 0
+// FIXME: this function needs to be updated to match BlendGammaSubpixel.
 void            FontRendererBlendGamma(FontRenderer *,
                     uint8_t *dst, int dst_stride,
                     uint8_t *src, int src_stride,
                     int region_width, int region_height,
                     FontRendererColor color);
+#endif
 
-void            FontRendererBlendGammaSubpixel(FontRenderer *,
-                    uint8_t *dst, int dst_stride,
-                    uint8_t *src, int src_stride,
-                    int region_width, int region_height,
-                    FontRendererColor color);
+void            FontRendererBlendGammaSubpixel(FontRenderer *font_renderer,
+                    FontRendererClipArea *clip, int x, int y,
+                    uint8_t *dst, int dst_width,
+                    const FontRendererBitmap *glyphs_bitmap,
+                    const GlyphBitmapInfo *glyph, FontRendererColor color);
 
 #ifdef __cplusplus
 }