Blend Environment glow before tonemapping and change default blend mode to screen.

Additionally, change the minimum `tonemap_white` parameter to `1.0`; users can increase `tonemap_exposure` for a similar effect to decreasing `tonemap_white` below `1.0`.

Co-authored-by: Hei <40064911+Lielay9@users.noreply.github.com>
Co-authored-by: Hugo Locurcio <hugo.locurcio@hugo.pro>

drivers/gles3/shaders/effects/post.glsl

@@ -45,6 +45,7 @@ uniform float luminance_multiplier;
 uniform sampler2D glow_color; // texunit:1
 uniform vec2 pixel_size;
 uniform float glow_intensity;
+uniform float srgb_white;
 
 vec4 get_glow_color(vec2 uv) {
 	vec2 half_pixel = pixel_size * 0.5;
@@ -58,6 +59,10 @@ vec4 get_glow_color(vec2 uv) {
 	color += textureLod(glow_color, uv + vec2(0.0, -half_pixel.y * 2.0), 0.0);
 	color += textureLod(glow_color, uv + vec2(-half_pixel.x, -half_pixel.y), 0.0) * 2.0;
 
+#ifdef USE_LUMINANCE_MULTIPLIER
+	color = color / luminance_multiplier;
+#endif
+
 	return color / 12.0;
 }
 #endif // USE_GLOW
@@ -102,18 +107,38 @@ void main() {
 	vec4 color = texture(source_color, uv_interp);
 #endif
 
-#ifdef USE_GLOW
-	vec4 glow = get_glow_color(uv_interp) * glow_intensity;
-
-	// Just use softlight...
-	glow.rgb = clamp(glow.rgb, vec3(0.0f), vec3(1.0f));
-	color.rgb = max((color.rgb + glow.rgb) - (color.rgb * glow.rgb), vec3(0.0));
-#endif // USE_GLOW
-
 #ifdef USE_LUMINANCE_MULTIPLIER
 	color = color / luminance_multiplier;
 #endif
 
+#ifdef USE_GLOW
+	// Glow blending is performed before srgb_to_linear because
+	// the glow texture was created from a nonlinear sRGB-encoded
+	// scene, so it only makes sense to add this glow to an equally
+	// nonlinear sRGB-encoded scene.
+
+	vec4 glow = get_glow_color(uv_interp) * glow_intensity;
+
+	// Glow always uses the screen blend mode in the Compatibility renderer:
+
+	// Glow cannot be above 1.0 after normalizing and should be non-negative
+	// to produce expected results. It is possible that glow can be negative
+	// if negative lights were used in the scene.
+	// We clamp to srgb_white because glow will be normalized to this range.
+	// Note: srgb_white cannot be smaller than the maximum output value (1.0).
+	glow.rgb = clamp(glow.rgb, 0.0, srgb_white);
+
+	// Normalize to srgb_white range.
+	//glow.rgb /= srgb_white;
+	//color.rgb /= srgb_white;
+	//color.rgb = (color.rgb + glow.rgb) - (color.rgb * glow.rgb);
+	// Expand back to original range.
+	//color.rgb *= srgb_white;
+
+	// The following is a mathematically simplified version of the above.
+	color.rgb = color.rgb + glow.rgb - (color.rgb * glow.rgb / srgb_white);
+#endif // USE_GLOW
+
 	color.rgb = srgb_to_linear(color.rgb);
 	color.rgb = apply_tonemapping(color.rgb, white);
 	color.rgb = linear_to_srgb(color.rgb);