X-Git-Url: https://git.sesse.net/?a=blobdiff_plain;f=sobel.frag;h=c4dabd519712465a9d2a5495617ac9c1e2708c5b;hb=adf0740352b3caa51e67a28429a11d5b082f16a0;hp=e6b377f2a54fab04a99cef15549f2e4cec9cc567;hpb=23ac2e109b1f6abd1830420e47bfc3ffaf97b7df;p=nageru

diff --git a/sobel.frag b/sobel.frag
index e6b377f..c4dabd5 100644
--- 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;
@@ -40,4 +47,8 @@ void main()
 
 	gradients.x = (top_right + 2.0f * right + bottom_right) - (top_left + 2.0f * left + bottom_left);
 	gradients.y = (top_left + 2.0 * top + top_right) - (bottom_left + 2.0f * bottom + bottom_right);
+
+	// Normalize so that we have a normalized unit of intensity levels per pixel.
+	gradients.x *= 0.125;
+	gradients.y *= 0.125;
 }