diff --git a/DEVELOPMENT.md b/DEVELOPMENT.md
index 76726a3..933456e 100644
--- a/DEVELOPMENT.md
+++ b/DEVELOPMENT.md
@@ -94,8 +94,8 @@ make -f Makefile.dev release-arch
   - [ ] src 15 : ?
   - [x] fx 11 : spill
   - [x] fx 12 : game of life
-  - [ ] fx 13 : colorize range (pretty color ranges)
-  - [ ] fx 14 : ?
+  - [x] fx 13 : sobel
+  - [ ] fx 14 : colorize range (pretty color ranges)
   - [ ] fx 15 : ?
   - [x] frag 10 : move debug screen here
   - [ ] debug : add src/fx/A/B indicators
diff --git a/README.md b/README.md
index f13314c..79fb67d 100644
--- a/README.md
+++ b/README.md
@@ -153,9 +153,9 @@ options:
 |       | **9** | Isometric grid    | _Zoom_            | _Scroll_        | _Elevation_        | Lens             | _Lens limit_    | _Lens power_  | _Pre Zoom_     |
 | **3** | **A** | Video In 2 + Thru | _Hue_             | _Saturation_    | _Light_            | Spill            | _Bottom limit_  | _Top Limit_   | _Rotation_     |
 |       | **B** | Scales            | _Zoom_            | _Shape_         | _Ripples_          | Game Of Life     | _Pixel Size_    | _Ruleset/Threshold_ | _Feedback_               |
-|       | **C** | (same as 1)       |                   |                 |                    |                  | (same as 1)    |               |                |
-|       | **D** | (same as 2)       |                   |                 |                    |                  | (same as 2)    |               |                |
-|       | **E** | (same as 3)       |                   |                 |                    |                  | (same as 3)    |               |                |
+|       | **C** | (same as 6)       |                   |                 |                    | Sobel Filter    | _Horz. filter_   | _Vert. filter_ | _Filter Size_ |
+|       | **D** | (same as 7)       |                   |                 |                    | (same as 0)     |                  |               |                |
+|       | **E** | (same as 8)       |                   |                 |                    | (same as 1)     |                  |               |                |
 
 TODO update
 
diff --git a/default/inc_functions.glsl b/default/inc_functions.glsl
index 00cc9fb..2548eb5 100644
--- a/default/inc_functions.glsl
+++ b/default/inc_functions.glsl
@@ -273,6 +273,22 @@ vec4 reframe_b(sampler2D tex, vec2 uv)
     return texture(tex, uv);
 }
 
+vec4 kernel(sampler2D tex, vec2 uv, mat3x3 k, float spm)
+{
+    int x, y;
+    vec2 offset;
+    vec4 sum = vec4(0);
+    for (y = -1; y <= 1; ++y) {
+        for (x = -1; x <= 1; ++x) {
+            offset = vec2(x, y) * spm;
+            if (abs(k[x + 1][y + 1]) > 0) {
+                sum += k[x + 1][y + 1] * texture(tex, uv + offset);
+            }
+        }
+    }
+    return sum;
+}
+
 // BLUR
 
 float gaussian_weight(float x, float sigma)
diff --git a/default/inc_fx.glsl b/default/inc_fx.glsl
index 18f4dc6..5339a9d 100644
--- a/default/inc_fx.glsl
+++ b/default/inc_fx.glsl
@@ -367,7 +367,7 @@ subroutine(fx_stage_sub) vec4 fx_12(vec2 vUV, sampler2D previous, sampler2D feed
     vec3 c0 = texture(previous, uv0).xyz;
 
     vec2 uv2 = uv1;
-    float k1 = pow(2, 10 - floor(pixel_size * 6));
+    float k1 = pow(2, 9 - floor(pixel_size * 5));
     float p = 1 / k1;
     uv2 = round(uv2 * k1) / k1;
     vec3 c1 = mix(
@@ -458,10 +458,9 @@ subroutine(fx_stage_sub) vec4 fx_12(vec2 vUV, sampler2D previous, sampler2D feed
     return fx_master(c0, cout, seed, m0);
 }
 
-// TODO FX 13
+// FX 13 : Sobel
 subroutine(fx_stage_sub) vec4 fx_13(vec2 vUV, sampler2D previous, sampler2D feedback, int seed, vec3 b1, vec2 f1, vec3 b2, vec2 f2, vec3 b3, vec2 f3, vec3 m0)
 {
-    return fx_2(vUV, previous, feedback, seed, b1, f1, b2, f2, b3, f3, m0);
     // start
 
 	vec2 uv0 = vUV.st;
@@ -470,10 +469,20 @@ subroutine(fx_stage_sub) vec4 fx_13(vec2 vUV, sampler2D previous, sampler2D feed
 
     // controls
 
+    float sx = magic(f1, b1, seed + 10) * 2;
+    float sy = magic(f2, b2, seed + 20) * 2;
+    float disp = 0.005 * magic(f3, b3, seed + 30);
+
     // logic
-    
+
     vec3 c0 = texture(previous, uv0).xyz;
-    vec3 c = c0;
+
+    const mat3x3 sobelx = {{-1, -2, -1}, {0, 0, 0}, {1, 2, 1}};
+    const mat3x3 sobely = {{-1, 0, 1}, {-2, 0, 2}, {-1, 0, 1}};
+
+    mat3x3 sobel = sobelx * sx + sobely * sy;
+
+    vec3 c = abs(kernel(previous, uv0, sobel, disp).xyz);
 
     return fx_master(c0, c, seed, m0);
 }
@@ -481,7 +490,7 @@ subroutine(fx_stage_sub) vec4 fx_13(vec2 vUV, sampler2D previous, sampler2D feed
 // TODO FX 14
 subroutine(fx_stage_sub) vec4 fx_14(vec2 vUV, sampler2D previous, sampler2D feedback, int seed, vec3 b1, vec2 f1, vec3 b2, vec2 f2, vec3 b3, vec2 f3, vec3 m0)
 {
-    return fx_3(vUV, previous, feedback, seed, b1, f1, b2, f2, b3, f3, m0);
+    return fx_1(vUV, previous, feedback, seed, b1, f1, b2, f2, b3, f3, m0);
     // start
 
 	vec2 uv0 = vUV.st;
@@ -501,7 +510,7 @@ subroutine(fx_stage_sub) vec4 fx_14(vec2 vUV, sampler2D previous, sampler2D feed
 // TODO FX 15
 subroutine(fx_stage_sub) vec4 fx_15(vec2 vUV, sampler2D previous, sampler2D feedback, int seed, vec3 b1, vec2 f1, vec3 b2, vec2 f2, vec3 b3, vec2 f3, vec3 m0)
 {
-    return fx_4(vUV, previous, feedback, seed, b1, f1, b2, f2, b3, f3, m0);
+    return fx_2(vUV, previous, feedback, seed, b1, f1, b2, f2, b3, f3, m0);
     // start
 
 	vec2 uv0 = vUV.st;
diff --git a/default/inc_src.glsl b/default/inc_src.glsl
index f3c12b0..f367d0b 100644
--- a/default/inc_src.glsl
+++ b/default/inc_src.glsl
@@ -175,6 +175,10 @@ subroutine(src_stage_sub) vec4 src_5(vec2 vUV, int seed, vec3 b1, vec2 f1, vec3
 // SRC 6 : video in 1 + thru
 subroutine(src_stage_sub) vec4 src_6(vec2 vUV, int seed, vec3 b1, vec2 f1, vec3 b2, vec2 f2, vec3 b3, vec2 f3)
 {
+    if (iDemo > 0) {
+        return src_2(vUV, seed, b1, f1, b2, f2, b3, f3);
+    }
+
     return src_thru(vUV, iTex3, seed, b1, f1, b2, f2, b3, f3);
 }
 
@@ -335,6 +339,10 @@ subroutine(src_stage_sub) vec4 src_10(vec2 vUV, int seed, vec3 b1, vec2 f1, vec3
 // SRC 11 : video in 2 + thru
 subroutine(src_stage_sub) vec4 src_11(vec2 vUV, int seed, vec3 b1, vec2 f1, vec3 b2, vec2 f2, vec3 b3, vec2 f3)
 {
+    if (iDemo > 0) {
+        return src_3(vUV, seed, b1, f1, b2, f2, b3, f3);
+    }
+
     return src_thru(vUV, iTex4, seed, b1, f1, b2, f2, b3, f3);
 }
 
@@ -384,7 +392,7 @@ subroutine(src_stage_sub) vec4 src_12(vec2 vUV, int seed, vec3 b1, vec2 f1, vec3
 // TODO SRC 13
 subroutine(src_stage_sub) vec4 src_13(vec2 vUV, int seed, vec3 b1, vec2 f1, vec3 b2, vec2 f2, vec3 b3, vec2 f3)
 {
-    return src_2(vUV, seed, b1, f1, b2, f2, b3, f3);
+    return src_7(vUV, seed, b1, f1, b2, f2, b3, f3);
     // start
 
 	vec2 uv0 = vUV.st;
@@ -401,7 +409,7 @@ subroutine(src_stage_sub) vec4 src_13(vec2 vUV, int seed, vec3 b1, vec2 f1, vec3
 // TODO SRC 14
 subroutine(src_stage_sub) vec4 src_14(vec2 vUV, int seed, vec3 b1, vec2 f1, vec3 b2, vec2 f2, vec3 b3, vec2 f3)
 {
-    return src_3(vUV, seed, b1, f1, b2, f2, b3, f3);
+    return src_8(vUV, seed, b1, f1, b2, f2, b3, f3);
     // start
 
 	vec2 uv0 = vUV.st;
@@ -418,7 +426,7 @@ subroutine(src_stage_sub) vec4 src_14(vec2 vUV, int seed, vec3 b1, vec2 f1, vec3
 // TODO SRC 15
 subroutine(src_stage_sub) vec4 src_15(vec2 vUV, int seed, vec3 b1, vec2 f1, vec3 b2, vec2 f2, vec3 b3, vec2 f3)
 {
-    return src_4(vUV, seed, b1, f1, b2, f2, b3, f3);
+    return src_9(vUV, seed, b1, f1, b2, f2, b3, f3);
     // start
 
 	vec2 uv0 = vUV.st;