Fix T47639: OpenGL render with smoke and fire incorrect when using

transparency.

The issue is that we are rendering to a 0..1 clamped sRGB buffer with
unpremultiplied alpha, where the correct thing to do would be to render
to an unclamped linear premultiplied alpha buffer. Then we would just
make fire purely emissive without affecting the alpha channel at all,
but that doesn't work here.

So for now, draw fire and smoke separately using different shaders and
blend modes, like it used to before the smoke programs were rewritten
(see rB0372b642).

2 files changed, 17 insertions, 12 deletions

diff --git a/source/blender/gpu/shaders/gpu_shader_fire_frag.glsl b/source/blender/gpu/shaders/gpu_shader_fire_frag.glsl
new file mode 100644
index 00000000000..3819203bcd9
--- /dev/null
+++ b/source/blender/gpu/shaders/gpu_shader_fire_frag.glsl
@@ -0,0 +1,17 @@
+
+varying vec3 coords;
+
+uniform sampler3D flame_texture;
+uniform sampler1D spectrum_texture;
+
+void main()
+{
+	float flame = texture3D(flame_texture, coords).r;
+	vec4 emission = texture1D(spectrum_texture, flame);
+
+	vec4 color;
+	color.rgb = emission.a * emission.rgb;
+	color.a = emission.a;
+
+	gl_FragColor = color;
+}
diff --git a/source/blender/gpu/shaders/gpu_shader_smoke_frag.glsl b/source/blender/gpu/shaders/gpu_shader_smoke_frag.glsl
index 4d1feb5c83e..fd790009e02 100644
--- a/source/blender/gpu/shaders/gpu_shader_smoke_frag.glsl
+++ b/source/blender/gpu/shaders/gpu_shader_smoke_frag.glsl
@@ -8,11 +8,6 @@ uniform float density_scale;
 uniform sampler3D soot_texture;
 uniform sampler3D shadow_texture;
 
-#ifdef USE_FIRE
-uniform sampler3D flame_texture;
-uniform sampler1D spectrum_texture;
-#endif
-
 void main()
 {
 	/* compute color and density from volume texture */
@@ -37,12 +32,5 @@ void main()
 	/* premultiply alpha */
 	vec4 color = vec4(soot_alpha * soot_color, soot_alpha);
 
-#ifdef USE_FIRE
-	/* fire */
-	float flame = texture3D(flame_texture, coords).r;
-	vec4 emission = texture1D(spectrum_texture, flame);
-	color.rgb += (1 - color.a) * emission.a * emission.rgb;
-#endif
-
 	gl_FragColor = color;
 }