Update some comments.

[nageru] / sobel.frag

diff --git a/sobel.frag b/sobel.frag
index e6b377f2a54fab04a99cef15549f2e4cec9cc567..9feab1ee6b26ba5c614fd5f0ad2cc920e31f58eb 100644 (file)
--- a/sobel.frag
+++ b/sobel.frag
@@ -4,28 +4,35 @@ in vec2 tc;
 out vec2 gradients;
 
 uniform sampler2D tex;
-uniform float inv_width, inv_height;
+uniform vec2 inv_image_size;
 
 void main()
 {
        // There are two common Sobel filters, horizontal and vertical
        // (see e.g. Wikipedia, or the OpenCV documentation):
        //
-       //  [-1 0 1]     [ 1  2  1]
-       //  [-2 0 2]     [ 0  0  0]
-       //  [-1 0 1]     [-1 -2 -1]
+       //  [1 0 -1]     [-1 -2 -1]
+       //  [2 0 -2]     [ 0  0  0]
+       //  [1 0 -1]     [ 1  2  1]
        // Horizontal     Vertical
        //
-       // Computing both at once allows us to get away with eight
+       // Note that Wikipedia and OpenCV gives entirely opposite definitions
+       // with regards to sign! This appears to be an error in the OpenCV
+       // documentation, forgetting that for convolution, the filters must be
+       // flipped. We have to flip the vertical matrix again comparing to
+       // Wikipedia, though, since we have bottom-left origin (y = up)
+       // and they define y as pointing downwards.
+       //
+       // Computing both directions at once allows us to get away with eight
        // texture samples instead of twelve.
 
-       float x_left   = tc.x - inv_width;
+       float x_left   = tc.x - inv_image_size.x;
        float x_mid    = tc.x; 
-       float x_right  = tc.x + inv_width;
+       float x_right  = tc.x + inv_image_size.x;
 
-       float y_top    = tc.y + inv_height;  // Note the bottom-left coordinate system.
+       float y_top    = tc.y + inv_image_size.y;  // Note the bottom-left coordinate system.
        float y_mid    = tc.y;
-       float y_bottom = tc.y - inv_height;
+       float y_bottom = tc.y - inv_image_size.y;
  
        float top_left     = texture(tex, vec2(x_left,  y_top)).x;
        float left         = texture(tex, vec2(x_left,  y_mid)).x;