sub-pixel rectangles with analytical anti-aliasing

src/standalone_srgb_antialiasing_experiments.c

@@ -76,6 +76,10 @@ typedef struct Vertex{
   F32 cg;
   F32 cb;
   F32 ca;
+  F32 rx0;
+  F32 ry0;
+  F32 rx1;
+  F32 ry1;
 } Vertex;
 
 
@@ -84,6 +88,7 @@ static void opengl_render_init(void);
 static void opengl_draw_basic_geometry(Vertex *v, U64 count);
 static void opengl_draw_srgb_in_geometry(Vertex *v, U64 count);
 static void opengl_draw_texture_geometry(Vertex *v, U64 count, U32 texture);
+static void opengl_draw_rect_geometry(Vertex *v, U64 count);
 
 
 ////////////////////////////////
@@ -297,6 +302,7 @@ static GLint  basic_u_view_xform = -1;
 
 
 
+
 static char srgb_in_vshader[] =
 "#version 330\n"
 "\n"
@@ -355,6 +361,45 @@ static GLint  srgb_in_u_view_xform = -1;
 
 
 
+
+static char rect_vshader[] =
+"#version 330\n"
+"uniform vec2 u_view_xform;\n"
+"layout (location = 0) in vec2 v_p;\n"
+"layout (location = 1) in vec4 v_r;\n"
+"out vec4 f_r;\n"
+"void main(){\n"
+"vec2 norm_pos = v_p*u_view_xform + vec2(-1.0, -1.0);\n"
+"gl_Position = vec4(norm_pos, 0.0, 1.0);\n"
+"f_r = v_r;\n"
+"}\n"
+;
+
+static char rect_fshader[] =
+"#version 330\n"
+"in vec4 gl_FragCoord;\n"
+"in vec4 f_r;\n"
+"out vec4 out_color;\n"
+"void main(){\n"
+"float pix_x0 = gl_FragCoord.x - 0.5;\n"
+"float pix_y0 = gl_FragCoord.y - 0.5;\n"
+"float pix_x1 = gl_FragCoord.x + 0.5;\n"
+"float pix_y1 = gl_FragCoord.y + 0.5;\n"
+"float int_x0 = max(pix_x0, f_r.x);\n"
+"float int_y0 = max(pix_y0, f_r.y);\n"
+"float int_x1 = min(pix_x1, f_r.z);\n"
+"float int_y1 = min(pix_y1, f_r.w);\n"
+"float cover = (int_x1 - int_x0)*(int_y1 - int_y0);\n"
+"out_color = vec4(1, 1, 1, cover);\n"
+"}\n"
+;
+
+static GLuint rect_program = 0;
+static GLint  rect_u_view_xform = -1;
+
+
+
+
 static char texture_vshader[] =
 "#version 330\n"
 "uniform vec2 u_view_xform;\n"
@@ -409,6 +454,15 @@ opengl_render_init(void){
     srgb_in_u_view_xform = glGetUniformLocation(basic_program, "u_view_xform");
   }
   
+  //- setup rect shader program
+  {
+    GLuint shaders[2] = {0};
+    shaders[0] = opengl_helper_make_shader(rect_vshader, GL_VERTEX_SHADER);
+    shaders[1] = opengl_helper_make_shader(rect_fshader, GL_FRAGMENT_SHADER);
+    rect_program = opengl_helper_make_program(shaders, 2);
+    rect_u_view_xform = glGetUniformLocation(rect_program, "u_view_xform");
+  }
+  
   //- setup texture shader program
   {
     GLuint shaders[2] = {0};
@@ -523,6 +577,24 @@ opengl_draw_srgb_in_geometry(Vertex *v, U64 count){
   glDrawArrays(GL_TRIANGLES, 0, count);
 }
 
+static void
+opengl_draw_rect_geometry(Vertex *v, U64 count){
+  glBufferData(GL_ARRAY_BUFFER, sizeof(*v)*count, v, GL_STREAM_DRAW);
+  
+  glUseProgram(rect_program);
+  glUniform2f(rect_u_view_xform, 2.f/graphics_w, 2.f/graphics_h);
+  
+  glEnableVertexAttribArray(0);
+  glVertexAttribPointer(0, 2, GL_FLOAT, 0,
+                        sizeof(Vertex), PtrOffsetOf(Vertex, px));
+  
+  glEnableVertexAttribArray(1);
+  glVertexAttribPointer(1, 4, GL_FLOAT, 0,
+                        sizeof(Vertex), PtrOffsetOf(Vertex, rx0));
+  
+  glDrawArrays(GL_TRIANGLES, 0, count);
+}
+
 static void
 opengl_draw_texture_geometry(Vertex *v, U64 count, U32 texture){
   glBufferData(GL_ARRAY_BUFFER, sizeof(*v)*count, v, GL_STREAM_DRAW);
@@ -602,6 +674,7 @@ WinMain(HINSTANCE hInstance,
     F32 test3_y = graphics_h - 110.f;
     F32 test4_y = graphics_h - 160.f;
     F32 test5_y = graphics_h - 340.f;
+    F32 test6_y = graphics_h - 450.f;
     
     // render to canvas frame buffer
     {
@@ -868,7 +941,40 @@ WinMain(HINSTANCE hInstance,
               opengl_draw_srgb_in_geometry(v, 6);
             }
           }
+        }
         
+        // 6: sub-pixel rectangle anti-aliasing
+        {
+          F32 top_y = test6_y;
+          F32 bot_y = test6_y - 20.f;
+          
+          F32 x_cursor = grad_x0;
+          for (U32 i = 0; i < 16; i += 1){
+            F32 l = x_cursor + 4.0625*i;
+            F32 r = l + 2.f;
+            
+            F32 floor_l = (F32)(S32)l;
+            F32 ceil_r  = (F32)(S32)r;
+            if (ceil_r < (F32)r){
+              ceil_r += 1.f;
+            }
+            
+            Vertex v[6];
+            v[0].px = floor_l; v[0].py = bot_y;
+            v[1].px = floor_l; v[1].py = top_y;
+            v[2].px = ceil_r;  v[2].py = bot_y;
+            v[3].px = floor_l; v[3].py = top_y;
+            v[4].px = ceil_r;  v[4].py = bot_y;
+            v[5].px = ceil_r;  v[5].py = top_y;
+            for (U32 j = 0; j < 6; j += 1){
+              v[j].rx0 = l;
+              v[j].ry0 = bot_y;
+              v[j].rx1 = r;
+              v[j].ry1 = top_y;
+            }
+            
+            opengl_draw_rect_geometry(v, 6);
+          }
         }
       }