Gave the flames it's own framebuffer to be able to manipulate it's color.

data/shader/flame.fsh

@@ -9,7 +9,7 @@ uniform bool withColor;
 
 void main() {
     if (withColor) {
-        oColor = vec4(fColor, 0.6);
+        oColor = vec4(fColor, 1.0);
     }
     else {
         oColor = vec4(0.0, 0.0, 0.0, 0.0);

data/shader/merge.fsh

@@ -0,0 +1,18 @@
+#version 150
+
+in vec2 vTexCoord;
+
+uniform sampler2D flame_fbo;
+uniform sampler2D light_fbo;
+
+out vec4 oColor;
+
+void main() {
+    vec4 flameColor = texture(flame_fbo, vTexCoord);
+    if (flameColor == vec4(0.0, 0.0, 0.0, 1.0)) {
+        oColor = texture(light_fbo, vTexCoord);
+    }
+    else {
+        oColor = mix(flameColor, texture(light_fbo, vTexCoord), 0.4);
+    }
+}

data/shader/merge.vsh

@@ -0,0 +1,11 @@
+#version 150
+
+in vec2 aPosition;
+in vec2 aTexCoord;
+
+out vec2 vTexCoord;
+
+void main() {
+    vTexCoord = aTexCoord;
+    gl_Position = vec4(aPosition, 0.0, 1.0);
+}

graphics.cc

@@ -32,6 +32,7 @@ void Graphics::init(Level* level) {
     glClearColor( 0.0, 0.0, 0.0, 1.0 );
     glEnable( GL_DEPTH_TEST );
     glEnable(GL_BLEND);
+    glBlendEquation(GL_FUNC_ADD);
     glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
     glEnable(GL_TEXTURE_CUBE_MAP_SEAMLESS);
     //glEnable(GL_MULTISAMPLE);
@@ -92,11 +93,14 @@ void Graphics::init(Level* level) {
     flamePostShader = ShaderProgramCreator("flame_post")
         .attributeLocations(fullscreen_quad->getAttributeLocations()).create();
 
+    mergeShader = ShaderProgramCreator("merge")
+        .attributeLocations(fullscreen_quad->getAttributeLocations()).create();
+
     glGetIntegerv(GL_MAX_COMBINED_TEXTURE_IMAGE_UNITS, &number_of_texture_units);
     printf("Your graphics card supports %d texture units.\n", number_of_texture_units);
     // Exit if we need more texture units
-    if (number_of_texture_units < 16) {
-        printf("You need at least 16 texture units to run this application. Exiting\n");
+    if (number_of_texture_units < 18) {
+        printf("You need at least 18 texture units to run this application. Exiting\n");
         exit(-1);
     }
 
@@ -169,6 +173,21 @@ void Graphics::init(Level* level) {
     skydomeShader->use();
     skydomeShader->setTexture("nightTexture", level->getSkydome()->getNightTexture()->getReference(), 15);
 
+    flame_fbo_color_texture = SharedTexture2D(new Texture2D(windowSize, GL_RGBA8));
+    flame_fbo_color_texture->setMinFilter(GL_NEAREST);
+    flame_fbo_color_texture->setMagFilter(GL_NEAREST);
+    flame_fbo_color_texture->setWrapS(GL_CLAMP_TO_BORDER);
+    flame_fbo_color_texture->setWrapT(GL_CLAMP_TO_BORDER);
+    framebuffer_flame = SharedFrameBufferObject(new FrameBufferObject());
+    framebuffer_flame->attachColorTexture("oColor", flame_fbo_color_texture);
+    framebuffer_flame->setDepthTexture(light_fbo_depth_texture);
+    framebuffer_flame->setClearColor(glm::vec4(0.0f, 0.0f, 0.0f, 0.0f));
+    framebuffer_flame->validate();
+
+    mergeShader->use();
+    mergeShader->setTexture("flame_fbo", flame_fbo_color_texture, 16);
+    mergeShader->setTexture("light_fbo", light_fbo_color_texture, 17);
+
     updateClosestLights();
 }
 
@@ -298,6 +317,8 @@ void Graphics::render(double time)
 
     // draw flames on top
     flameShader->use();
+    framebuffer_flame->bind();
+    glClear(GL_COLOR_BUFFER_BIT);
     // cull faces to get consistent color while using alpha
     glEnable(GL_CULL_FACE);
     glCullFace(GL_BACK);
@@ -317,10 +338,18 @@ void Graphics::render(double time)
     flame_positions->render();
     flameShader->setUniform("left", false);
     flame_positions->render();
+    glDisable(GL_CULL_FACE);
 
+    framebuffer_light->bind();
     glDepthMask(GL_FALSE);
+    mergeShader->use();
+    glDisable(GL_DEPTH_TEST);
+    fullscreen_quad->render();
+    glEnable(GL_DEPTH_TEST);
+
 
     // draw slightly larger only for stencil buffer to blur edges
+    flameShader->use();
     glEnable(GL_STENCIL_TEST);
     glStencilFunc(GL_ALWAYS, 1, 0xFF); //Set any stencil to 1
     glStencilOp(GL_KEEP, GL_KEEP, GL_REPLACE);
@@ -346,11 +375,9 @@ void Graphics::render(double time)
     glStencilFunc(GL_EQUAL, 1, 0xFF); //Pass test if stencil value is 1
     glStencilMask(0x00);// don't write to stencil buffer
 
-    glDepthMask(GL_TRUE);
-    glDisable(GL_CULL_FACE);
-
     flamePostShader->use();
     fullscreen_quad->render();
+    glDepthMask(GL_TRUE);
 
     glDisable(GL_STENCIL_TEST);
 

graphics.hh

@@ -36,11 +36,14 @@ class Graphics {
         SharedShaderProgram depthShader;
         SharedShaderProgram flameShader;
         SharedShaderProgram flamePostShader;
+        SharedShaderProgram mergeShader;
         std::vector<SharedTexture2D> depth_directionalMaps;
         std::vector<SharedFrameBufferObject> framebuffer_directional;
         std::vector<SharedTextureCubeMap> depth_cubeMaps;
         SharedFrameBufferObject framebuffer_cube;
         SharedFrameBufferObject framebuffer_light;
+        SharedFrameBufferObject framebuffer_flame;
+        SharedTexture2D flame_fbo_color_texture;
         SharedTexture2D light_fbo_color_texture;
         SharedTexture2D light_fbo_depth_texture;
         SharedVertexArrayObject flame_positions;