Fireflies on terrain

[TokisanGames]

Description

With a light background and dim light, it's possible to see white dots or fireflies from the background through the terrain, even though the meshes are perfectly aligned. It changes based on camera position, angle, and viewport aspect ratio.

This issue was discovered and worked around in Godot 3. I hadn't been able to reproduce it in GD4 until today. It is easily reproducable in the demo by turning off the light and brightening up the ground color in Sky and moving the camera around.

I was most strongly seeing it along the highlighted areas.

Fortunately, the same fix works.
godotengine/godot#35067 (comment)

It's an issue in the godot renderer. The most effective solution is to avoid using the default model space to view space transform in one operation (MODELVIEW_MATRIX) by adding the skip_vertex_transform render mode. Then we do our own conversion in two separate steps at the end of vertex().

	VERTEX = (MODEL_MATRIX * vec4(VERTEX, 1.0)).xyz;
	VERTEX = (VIEW_MATRIX * vec4(VERTEX, 1.0)).xyz;

cc: @Xtarsia

[TokisanGames]

Fixed in 1a312cc and 336f462

[TokisanGames]

I discovered this fix introduces striations to the Normal shadows. It also subtly responds to camera angle. This page says more is needed:
https://docs.godotengine.org/en/4.2/tutorials/shaders/shader_reference/spatial_shader.html#vertex-built-ins

NORMAL = normalize((MODELVIEW_MATRIX * vec4(NORMAL, 0.0)).xyz);
BINORMAL = normalize((MODELVIEW_MATRIX * vec4(BINORMAL, 0.0)).xyz); //optional
TANGENT = normalize((MODELVIEW_MATRIX * vec4(TANGENT, 0.0)).xyz); //optional

Only the Normal line is needed, but I added all. This gives the identical visual appearance as before, however it is 0.10ms faster, or 10-40fps! I added those and improved the lighting by blending the splits, which took a little performance.

Fixed in f5f676f