From a41754b382fef8538a2497a6a9ae1febd235c46d Mon Sep 17 00:00:00 2001
From: Francesco Abbate <francesco.bbt@gmail.com>
Date: Fri, 11 Dec 2020 14:36:32 +0100
Subject: [PATCH] Fix font hinting when using subpixel rendering

Ensure we set the font's size with freetype to the intended size so that
hinting works as expected and perform subpixel scaling using affine
matrix coefficient.
---
 lib/font_renderer/font_renderer_alpha.h | 19 ++++++++-----------
 1 file changed, 8 insertions(+), 11 deletions(-)

diff --git a/lib/font_renderer/font_renderer_alpha.h b/lib/font_renderer/font_renderer_alpha.h
index 5dcf5e37..53304688 100644
--- a/lib/font_renderer/font_renderer_alpha.h
+++ b/lib/font_renderer/font_renderer_alpha.h
@@ -81,20 +81,17 @@ public:
     void set_font_height(double height) {
         const double scale_x = (m_prescale_x ? 100.0 : 1.0);
         m_feng.height(height);
-        if (m_subpixel) {
-            const int subpixel_scale = 3;
-            m_feng.width(height * scale_x * subpixel_scale);
-        } else {
-            m_feng.width(height * scale_x);
-        }
+        m_feng.width(height * scale_x);
     }
 
     template<class Rasterizer, class Scanline, class RenSolid>
     void draw_codepoint(Rasterizer& ras, Scanline& sl,
         RenSolid& ren_solid, const color_type color,
-        int codepoint, double& x, double& y, int subpixel_scale)
+        int codepoint, double& x, double& y, const int subpixel_scale)
     {
         const double scale_x = (m_prescale_x ? 100.0 : 1.0);
+        // Coefficient to scale back the glyph to the final scale.
+        const double cx_inv_scale = subpixel_scale / scale_x;
 
         // Represent the delta in x scaled by scale_x.
         double x_delta = 0;
@@ -114,15 +111,15 @@ public:
                 double ty = m_hinting ? floor(y + 0.5) : y;
                 ras.reset();
                 m_mtx.reset();
-                m_mtx *= agg::trans_affine_scaling(1.0 / scale_x, 1);
-                m_mtx *= agg::trans_affine_translation(start_x + x_delta / scale_x, ty);
+                m_mtx *= agg::trans_affine_scaling(cx_inv_scale, 1);
+                m_mtx *= agg::trans_affine_translation(start_x + cx_inv_scale * x_delta, ty);
                 ras.add_path(m_trans);
                 ren_solid.color(color);
                 agg::render_scanlines(ras, sl, ren_solid);
             }
 
             y += glyph->advance_y;
-            x += (x_delta + glyph->advance_x) / scale_x;
+            x += cx_inv_scale * (x_delta + glyph->advance_x);
         }
     }
 
@@ -135,7 +132,7 @@ public:
     void render_codepoint(agg::rendering_buffer& ren_buf,
         const color_type text_color,
         double& x, double& y,
-        int codepoint, int subpixel_scale)
+        int codepoint, const int subpixel_scale)
     {
         if (!m_font_loaded) {
             return;