draw a shaded quad, track mouse

example.rb

@@ -17,12 +17,16 @@ class Example
   def load_shaders
     v_id = glCreateShader(GL_VERTEX_SHADER)
     glShaderSource(v_id, <<-EOS)
+      uniform vec2 offset;
+      uniform vec2 screen_size;
       attribute vec2 pos;
       varying vec4 pos_i;
 
       void main()
       {
-        pos_i = vec4(pos, 0, 1);
+        vec2 scaled_pos = vec2((offset.x - screen_size.x / 2) / screen_size.x * 2,
+                               (offset.y - screen_size.y / 2) / screen_size.y * 2);
+        pos_i = vec4(pos + scaled_pos, 0, 1);
         gl_Position = pos_i;
       }
     EOS
@@ -37,7 +41,9 @@ class Example
 
       void main()
       {
-        gl_FragColor = pos_i;
+        gl_FragColor = vec4((pos_i.x + 1.0) / 2.0,
+                            (pos_i.y + 1.0) / 2.0,
+                            0, 1);
       }
     EOS
     glCompileShader(f_id)
@@ -52,6 +58,8 @@ class Example
     unless glGetProgramiv(p_id, GL_LINK_STATUS)
       raise glGetProgramInfoLog(p_id)
     end
+    $offset_loc = glGetUniformLocation(p_id, "offset")
+    $screen_size_loc = glGetUniformLocation(p_id, "screen_size")
 
     glUseProgram(p_id)
   end
@@ -68,6 +76,15 @@ class Example
   end
 
   def run
+    quad_attrs = [0.02, 0.02,
+                  -0.02, 0.02,
+                  -0.02, -0.02,
+                  0.02, -0.02].pack("f*")
+    b_id = glGenBuffers(1).first
+    glBindBuffer(GL_ARRAY_BUFFER, b_id);
+    glBufferData(GL_ARRAY_BUFFER, 2 * 4 * 4, quad_attrs, GL_STATIC_DRAW);
+
+    x, y = 0, 0
     while @window.open?
       @window.each_event do |event|
         case event.type
@@ -81,13 +98,25 @@ class Example
             @fullscreen = !@fullscreen
             create_window
           end
+        when Event::MouseMoved
+          x = event.mouse_move.x
+          y = event.mouse_move.y
         when Event::Resized # Resize event : adjust viewport
+          x = event.size.width / 2
+          y = event.size.height / 2
           glViewport(0, 0, event.size.width, event.size.height)
         end
       end
+      break unless @window.open?
 
       glClear(GL_COLOR_BUFFER_BIT)
 
+      glUniform2f($offset_loc, x, @window.getSize.y - y)
+      glUniform2f($screen_size_loc, @window.getSize.x, @window.getSize.y)
+      glEnableVertexAttribArray(0)
+      glVertexAttribPointer(0, 2, GL_FLOAT, GL_FALSE, 2 * 4, 0);
+      glDrawArrays(GL_TRIANGLE_FAN, 0, 4);
+
       @window.display
     end
   end