Use screen blending with alpha compositing for LightWidget rendering

include/app.hpp

@@ -408,8 +408,9 @@ struct MidiWidget : LedDisplay {
 ////////////////////
 
 struct LightWidget : TransparentWidget {
-	NVGcolor borderColor = nvgRGBA(0, 0, 0, 0);
+	NVGcolor bgColor = nvgRGBA(0, 0, 0, 0);
 	NVGcolor color = nvgRGBA(0, 0, 0, 0);
+	NVGcolor borderColor = nvgRGBA(0, 0, 0, 0);
 	void draw(NVGcontext *vg) override;
 	virtual void drawLight(NVGcontext *vg);
 	virtual void drawHalo(NVGcontext *vg);
@@ -417,8 +418,6 @@ struct LightWidget : TransparentWidget {
 
 /** Mixes a list of colors based on a list of brightness values */
 struct MultiLightWidget : LightWidget {
-	/** Color of the "off" state */
-	NVGcolor bgColor = nvgRGBA(0, 0, 0, 0);
 	/** Colors of each value state */
 	std::vector<NVGcolor> baseColors;
 	void addBaseColor(NVGcolor baseColor);

include/util/color.hpp

@@ -26,11 +26,33 @@ inline NVGcolor colorPlus(NVGcolor a, NVGcolor b) {
 	return a;
 }
 
+inline NVGcolor colorMult(NVGcolor a, NVGcolor b) {
+	for (int i = 0; i < 3; i++)
+		a.rgba[i] *= b.rgba[i];
+	return a;
+}
+
 inline NVGcolor colorMult(NVGcolor a, float x) {
 	for (int i = 0; i < 3; i++)
 		a.rgba[i] *= x;
 	return a;
 }
 
+/** Screen blending with alpha compositing */
+inline NVGcolor colorScreen(NVGcolor a, NVGcolor b) {
+	if (a.a == 0.0)
+		return b;
+	if (b.a == 0.0)
+		return a;
+
+	a = colorMult(a, a.a);
+	b = colorMult(b, b.a);
+	NVGcolor c = colorMinus(colorPlus(a, b), colorMult(a, b));
+	c.a = a.a + b.a - a.a * b.a;
+	c = colorMult(c, 1.f / c.a);
+	c = colorClip(c);
+	return c;
+}
+
 
 } // namespace rack

src/app/LightWidget.cpp

@@ -16,7 +16,11 @@ void LightWidget::drawLight(NVGcontext *vg) {
 	nvgBeginPath(vg);
 	nvgCircle(vg, radius, radius, radius);
 
-	// Solid color
+	// Background
+	nvgFillColor(vg, bgColor);
+	nvgFill(vg);
+
+	// Foreground
 	nvgFillColor(vg, color);
 	nvgFill(vg);
 
@@ -28,13 +32,13 @@ void LightWidget::drawLight(NVGcontext *vg) {
 
 void LightWidget::drawHalo(NVGcontext *vg) {
 	float radius = box.size.x / 2.0;
-	float oradius = radius + 15.0;
+	float oradius = radius + 20.0;
 
 	nvgBeginPath(vg);
 	nvgRect(vg, radius - oradius, radius - oradius, 2*oradius, 2*oradius);
 
 	NVGpaint paint;
-	NVGcolor icol = colorMult(color, 0.25);
+	NVGcolor icol = colorMult(color, 0.10);
 	NVGcolor ocol = nvgRGB(0, 0, 0);
 	paint = nvgRadialGradient(vg, radius, radius, radius, oradius, icol, ocol);
 	nvgFillPaint(vg, paint);

src/app/MultiLightWidget.cpp

@@ -11,11 +11,11 @@ void MultiLightWidget::addBaseColor(NVGcolor baseColor) {
 
 void MultiLightWidget::setValues(const std::vector<float> &values) {
 	assert(values.size() == baseColors.size());
-	color = nvgRGBf(0, 0, 0);
+	color = nvgRGBAf(0, 0, 0, 0);
 	for (size_t i = 0; i < baseColors.size(); i++) {
 		NVGcolor c = baseColors[i];
-		c = colorMult(c, values[i]);
-		color = colorPlus(color, c);
+		c.a *= clamp(values[i], 0.f, 1.f);
+		color = colorScreen(color, c);
 	}
 	color = colorClip(color);
 }