3 files changed, 102 insertions, 5 deletions

diff --git a/src/shader/code/extra.glsl b/src/shader/code/extra.glsl
new file mode 100644
index 0000000..5ba6207
--- /dev/null
+++ b/src/shader/code/extra.glsl
@@ -0,0 +1,53 @@
+static const std::string EXTRA_SHADER_CODE = R"(
+#version 430
+
+layout (local_size_x = 1, local_size_y = 1) in;
+
+layout (std430, binding=3) buffer bufferFluid { float fluidCells[]; };
+layout (std430, binding=4) buffer bufferExtra { float extraCells[]; };
+
+uniform uint nX;
+uniform uint nY;
+
+const float convLength = 1.0 / float(max(nX,nY));
+
+/// Array indexing
+
+uint indexOfDirection(int i, int j) {
+	return 3*(i+1) + (j+1);
+}
+
+uint indexOfFluidVertex(uint x, uint y) {
+	return 3*nX*y + 3*x;
+}
+
+/// Data access
+
+vec2 getFluidVelocity(uint x, uint y) {
+	const uint idx = indexOfFluidVertex(x, y);
+	return vec2(
+		fluidCells[idx + 0],
+		fluidCells[idx + 1]
+	);
+}
+
+void setFluidExtra(uint x, uint y, float curl) {
+	const uint idx = indexOfFluidVertex(x, y);
+	extraCells[idx + 0] = curl;
+}
+
+void main() {
+	const uint x = gl_GlobalInvocationID.x;
+	const uint y = gl_GlobalInvocationID.y;
+
+	if ( !(x < nX && y < nY) ) {
+		return;
+	}
+
+	// simple central difference discretization of the 2d curl operator
+	const float dxvy = (getFluidVelocity(x+1,y).y - getFluidVelocity(x-1,y).y) / (2*convLength);
+	const float dyvx = (getFluidVelocity(x,y+1).x - getFluidVelocity(x,y-1).x) / (2*convLength);
+
+	setFluidExtra(x, y,	dxvy - dyvx);
+}
+)";
diff --git a/src/shader/code/interact.glsl b/src/shader/code/interact.glsl
index 613c4cd..141673a 100644
--- a/src/shader/code/interact.glsl
+++ b/src/shader/code/interact.glsl
@@ -4,6 +4,7 @@ static const std::string INTERACT_SHADER_CODE = R"(
 layout (local_size_x = 1, local_size_y = 1) in;
 
 layout (std430, binding=3) buffer bufferFluid { float fluidCells[]; };
+layout (std430, binding=4) buffer bufferExtra { float extraCells[]; };
 
 uniform uint nX;
 uniform uint nY;
@@ -62,6 +63,7 @@ int getMaterial(uint x, uint y) {
 void setMaterial(uint x, uint y, int m) {
 	const uint idx = indexOfFluidVertex(x, y);
 	fluidCells[idx + 2] = m;
+	extraCells[idx + 2] = m;
 }
 
 /// Geometry cleanup
diff --git a/src/shader/code/vertex.glsl b/src/shader/code/vertex.glsl
index 0f0e07c..5136023 100644
--- a/src/shader/code/vertex.glsl
+++ b/src/shader/code/vertex.glsl
@@ -11,6 +11,7 @@ uniform uint nX;
 uniform uint nY;
 
 uniform bool show_fluid_quality;
+uniform bool show_curl;
 uniform int  palette_factor;
 
 float unit(float x) {
@@ -65,6 +66,38 @@ vec3 trafficLightPalette(float x) {
 	}
 }
 
+vec3 blueBlackRedPalette(float x) {
+	if ( x < 0.5 ) {
+		return mix(
+			vec3(0.0, 0.0, 1.0),
+			vec3(0.0, 0.0, 0.0),
+			2*x
+		);
+	} else {
+		return mix(
+			vec3(0.0, 0.0, 0.0),
+			vec3(1.0, 0.0, 0.0),
+			2*(x - 0.5)
+		);
+	}
+}
+
+vec3 blackGoldPalette(float x) {
+	return mix(
+		vec3(0.0, 0.0, 0.0),
+		vec3(0.5, 0.35, 0.05),
+		x
+	);
+}
+
+vec3 blueRedPalette(float x) {
+	return mix(
+		vec3(0.0, 0.0, 1.0),
+		vec3(1.0, 0.0, 0.0),
+		x
+	);
+}
+
 void main() {
 	const vec2 idx = fluidVertexAtIndex(gl_VertexID);
 
@@ -83,12 +116,21 @@ void main() {
 		vs_out.color = vec3(0.0, 0.0, 0.0);
 	} else {
 		if ( show_fluid_quality ) {
-			vs_out.color = trafficLightPalette(restrictedQuality(VertexPosition.y));
+			vs_out.color = trafficLightPalette(
+				restrictedQuality(VertexPosition.y)
+			);
+		} else if ( show_curl ) {
+			const float factor = 1.0 / float(100*palette_factor);
+			if ( abs(VertexPosition.x) > 1.0 ) {
+				vs_out.color = blueBlackRedPalette(
+					0.5 + (VertexPosition.x * factor)
+				);
+			} else {
+				vs_out.color = vec3(0.0,0.0,0.0);
+			}
 		} else {
-			vs_out.color = mix(
-				vec3(-0.5, 0.0, 1.0),
-				vec3( 1.0, 0.0, 0.0),
-				norm(VertexPosition.xy) / float(palette_factor)
+			vs_out.color = blueRedPalette(
+				norm(VertexPosition.xy) / palette_factor
 			);
 		}
 	}