changed: Clarified the transform for non-linear stretching, and split actual stretching between linear and non-linear

git-svn-id: https://xbmc.svn.sourceforge.net/svnroot/xbmc/trunk@28416 568bbfeb-2a22-0410-94d2-cc84cf5bfa90

4 files changed, 16 insertions, 4 deletions

diff --git a/system/shaders/convolution-4x4.glsl b/system/shaders/convolution-4x4.glsl
index 62db84a976..7dc88f11f1 100644
--- a/system/shaders/convolution-4x4.glsl
+++ b/system/shaders/convolution-4x4.glsl
@@ -32,8 +32,11 @@ half4 weight(float pos)
   #define POSITION(pos) stretch(pos)
   vec2 stretch(vec2 pos)
   {
+    // our transform should map [0..1] to itself, with f(0) = 0, f(1) = 1, f(0.5) = 0.5, and f'(0.5) = b.
+    // a simple curve to do this is g(x) = b(x-0.5) + (1-b)2^(n-1)(x-0.5)^n + 0.5
+    // where the power preserves sign. n = 2 is the simplest non-linear case (required when b != 1)
     float x = pos.x - 0.5;
-    return vec2(mix(x, x * abs(x) * 2.0, m_stretch) + 0.5, pos.y);
+    return vec2(mix(x * abs(x) * 2.0, x, m_stretch) + 0.5, pos.y);
   }
 #else
   #define POSITION(pos) (pos)
diff --git a/system/shaders/convolution-6x6.glsl b/system/shaders/convolution-6x6.glsl
index 9197ebb34f..76ea02e24b 100644
--- a/system/shaders/convolution-6x6.glsl
+++ b/system/shaders/convolution-6x6.glsl
@@ -32,8 +32,11 @@ half3 weight(float pos)
   #define POSITION(pos) stretch(pos)
   vec2 stretch(vec2 pos)
   {
+    // our transform should map [0..1] to itself, with f(0) = 0, f(1) = 1, f(0.5) = 0.5, and f'(0.5) = b.
+    // a simple curve to do this is g(x) = b(x-0.5) + (1-b)2^(n-1)(x-0.5)^n + 0.5
+    // where the power preserves sign. n = 2 is the simplest non-linear case (required when b != 1)
     float x = pos.x - 0.5;
-    return vec2(mix(x, x * abs(x) * 2.0, m_stretch) + 0.5, pos.y);
+    return vec2(mix(x * abs(x) * 2.0, x, m_stretch) + 0.5, pos.y);
   }
 #else
   #define POSITION(pos) (pos)
diff --git a/system/shaders/stretch.glsl b/system/shaders/stretch.glsl
index 2e368563e0..ac7ddd9783 100644
--- a/system/shaders/stretch.glsl
+++ b/system/shaders/stretch.glsl
@@ -3,8 +3,11 @@ uniform float     m_stretch;
 
 vec2 stretch(vec2 pos)
 {
+  // our transform should map [0..1] to itself, with f(0) = 0, f(1) = 1, f(0.5) = 0.5, and f'(0.5) = b.
+  // a simple curve to do this is g(x) = b(x-0.5) + (1-b)2^(n-1)(x-0.5)^n + 0.5
+  // where the power preserves sign. n = 2 is the simplest non-linear case (required when b != 1)
   float x = pos.x - 0.5;
-  return vec2(mix(x, x * abs(x) * 2.0, m_stretch) + 0.5, pos.y);
+  return vec2(mix(x * abs(x) * 2.0, x, m_stretch) + 0.5, pos.y);
 }
 
 void main()
diff --git a/system/shaders/yuv2rgb_basic.glsl b/system/shaders/yuv2rgb_basic.glsl
index 6841d5f0f8..1527a7a4d1 100644
--- a/system/shaders/yuv2rgb_basic.glsl
+++ b/system/shaders/yuv2rgb_basic.glsl
@@ -19,8 +19,11 @@ uniform float     m_stretch;
   #define POSITION(pos) stretch(pos)
   vec2 stretch(vec2 pos)
   {
+    // our transform should map [0..1] to itself, with f(0) = 0, f(1) = 1, f(0.5) = 0.5, and f'(0.5) = b.
+    // a simple curve to do this is g(x) = b(x-0.5) + (1-b)2^(n-1)(x-0.5)^n + 0.5
+    // where the power preserves sign. n = 2 is the simplest non-linear case (required when b != 1)
     float x = pos.x - 0.5;
-    return vec2(mix(x, x * abs(x) * 2.0, m_stretch) + 0.5, pos.y);
+    return vec2(mix(2.0 * x * abs(x), x, m_stretch) + 0.5, pos.y);
   }
 #else
   #define POSITION(pos) (pos)