Rework patch placement. Finally inches our EPE just below the reference code, it...

[nageru] / motion_search.vert

diff --git a/motion_search.vert b/motion_search.vert
index dbb27956814ed3438b892dc9ed95256587746ebc..9e25e7a74c19356b448706aad0c7111ee4fc22f9 100644 (file)
--- a/motion_search.vert
+++ b/motion_search.vert
@@ -2,12 +2,31 @@
 
 in vec2 position;
 out vec2 flow_tc;
-out vec2 patch_bottom_left_texel;  // Center of bottom-left texel of patch.
+out vec2 patch_center;
 
-uniform vec2 inv_flow_size, inv_image_size;
+uniform vec2 flow_size;
 
 void main()
 {
+       // Patch placement: We want the outermost patches to have centers exactly in the
+       // image corners, so that the bottom-left patch has centre (0,0) and the
+       // upper-right patch has center (1,1). The position we get in is _almost_ there;
+       // since the quad's corners are in (0,0) and (1,1), the fragment shader will get
+       // centers in x=0.5/w, x=1.5/w and so on (and similar for y).
+       //
+       // In other words, find some f(x) = ax + b so that
+       //
+       //   a 0.5 / w + b = 0
+       //   a (1.0 - 0.5 / w) + b = 1
+       //
+       // which gives
+       //
+       //   a = 1 / (w - 1)
+       //   b = w / 2 (w - 1)
+       vec2 a = flow_size / (flow_size - 1);
+       vec2 b = -1.0 / (2 * (flow_size - 1.0));
+       patch_center = a * position + b;
+
        // The result of glOrtho(0.0, 1.0, 0.0, 1.0, 0.0, 1.0) is:
        //
        //   2.000  0.000  0.000 -1.000
@@ -16,7 +35,4 @@ void main()
        //   0.000  0.000  0.000  1.000
        gl_Position = vec4(2.0 * position.x - 1.0, 2.0 * position.y - 1.0, -1.0, 1.0);
        flow_tc = position;
-
-       vec2 patch_bottom_left = position - vec2(0.5, 0.5) * inv_flow_size;
-       patch_bottom_left_texel = patch_bottom_left + vec2(0.5, 0.5) * inv_image_size;
 }