Implement limb darkening for the Sun

data/shaders/planet.frag

@@ -185,6 +185,30 @@ vec3 linearToSRGB(vec3 lin)
 
 void main()
 {
+#ifndef IS_MOON
+	if(sunInfo.w==0.)
+	{
+		// We are drawing the Sun
+		vec4 texColor = texture2D(tex, texc);
+		texColor.rgb = srgbToLinear(texColor.rgb * sunInfo.rgb);
+		// Reference: chapter 14.7 "Limb Darkening" in "Allen’s Astrophysical Quantities",
+		//            A.N.Cox (ed.), 4th edition, New York: Springer-Verlag, 2002.
+		//            DOI 10.1007/978-1-4612-1186-0
+		// The values for u2 and v2 for wavelengths 400nm-800nm were taken, linearly
+		// interpolated, and integrated against CIE 1931 color matching functions.
+		// The results were transformed from XYZ to linear sRGB color space.
+		// We call the results for u2 "a1", and for v2 "a2".
+		const vec3 a2 = vec3(-0.226988526315793, -0.232934589453355, -0.153026433664999);
+		const vec3 a1 = vec3(0.848380336865573, 0.937696820066542, 0.981762186155682);
+		const vec3 a0 = vec3(1) - a1 - a2;
+		float cosTheta = dot(eyeDirection, normalize(normalVS));
+		float cosTheta2 = cosTheta*cosTheta;
+		vec3 limbDarkeningCoef = a0 + a1*cosTheta + a2*cosTheta2;
+		vec3 color = texColor.rgb * limbDarkeningCoef;
+		FRAG_COLOR = vec4(linearToSRGB(color), texColor.a);
+		return;
+	}
+#endif
     mediump float final_illumination = 1.0;
 #ifdef OREN_NAYAR
     mediump float lum = 1.;

src/core/modules/Planet.cpp

@@ -3882,10 +3882,13 @@ Planet::RenderData Planet::setCommonShaderUniforms(const StelPainter& painter, Q
 	GL(shader->setUniformValue(shaderVars.tex, 0));
 	GL(shader->setUniformValue(shaderVars.shadowCount, static_cast<GLint>(data.shadowCandidates.size())));
 	GL(shader->setUniformValue(shaderVars.shadowData, data.shadowCandidatesData));
-	GL(shader->setUniformValue(shaderVars.sunInfo, static_cast<GLfloat>(data.mTarget[12]),
-												   static_cast<GLfloat>(data.mTarget[13]),
-												   static_cast<GLfloat>(data.mTarget[14]),
-												   static_cast<GLfloat>(sun->getEquatorialRadius())));
+	if(this!=sun)
+	{
+		GL(shader->setUniformValue(shaderVars.sunInfo, static_cast<GLfloat>(data.mTarget[12]),
+													   static_cast<GLfloat>(data.mTarget[13]),
+													   static_cast<GLfloat>(data.mTarget[14]),
+													   static_cast<GLfloat>(sun->getEquatorialRadius())));
+	}
 	GL(shader->setUniformValue(shaderVars.skyBrightness, lmgr->getAtmosphereAverageLuminance()));
 	GL(shader->setUniformValue(shaderVars.poleLat, 1.1f, -0.1f)); // Avoid white objects. poleLat is only used for Mars.
 
@@ -3956,15 +3959,6 @@ void Planet::drawSphere(StelPainter* painter, float screenRd, bool drawOnlyRing)
 
 	const SolarSystem* ssm = GETSTELMODULE(SolarSystem);
 
-	if (this==ssm->getSun())
-	{
-		texMap->bind();
-		//painter->setColor(2, 2, 0.2); // This is now in draw3dModel() to apply extinction
-		painter->setArrays(reinterpret_cast<const Vec3f*>(projectedVertexArr.constData()), reinterpret_cast<const Vec2f*>(model.texCoordArr.constData()));
-		painter->drawFromArray(StelPainter::Triangles, model.indiceArr.size(), 0, false, model.indiceArr.constData());
-		return;
-	}
-
 	//cancel out if shaders are invalid
 	if(shaderError)
 		return;
@@ -4006,6 +4000,11 @@ void Planet::drawSphere(StelPainter* painter, float screenRd, bool drawOnlyRing)
 	GL(shader->bind());
 
 	RenderData rData = setCommonShaderUniforms(*painter,shader,*shaderVars);
+	if(this==ssm->getSun())
+	{
+		const auto color = painter->getColor();
+		GL(shader->setUniformValue(shaderVars->sunInfo, color[0], color[1], color[2], 0.f));
+	}
 
 	if (rings!=Q_NULLPTR)
 	{