Fix proximity fade in Compatibility renderer

[tetrapod00]

Fixes #89942.
Supersedes #89966.

Uses the correct NDC for the Compatibility renderer.

Uses the preprocessor defines for the current renderer introduced in 4.4 in #98549. We also document a nearly identical reconstruction of world space using NDC in this tutorial, which now uses the preprocessor defines to be renderer-independent.

[tetrapod00]

Alternately:

Suggested change

	vec4 proximity_view_pos = INV_PROJECTION_MATRIX * vec4(SCREEN_UV * 2.0 - 1.0, proximity_depth_tex * 2.0 - 1.0, 1.0);
	vec4 proximity_view_pos = INV_PROJECTION_MATRIX * vec4(vec3(SCREEN_UV, proximity_depth_tex) * 2.0 - 1.0, 1.0);

We could also do something more like this, leaving the "default" case of Forward+/Mobile implicit:

float proximity_depth_tex = textureLod(depth_texture, SCREEN_UV, 0.0).r;
#if CURRENT_RENDERER == RENDERER_COMPATIBILITY
proximity_depth_tex = proximity_depth_tex * 2.0 - 1.0;
#endif
vec4 proximity_view_pos = INV_PROJECTION_MATRIX * vec4(SCREEN_UV * 2.0 - 1.0, proximity_depth_tex, 1.0);

But I think the current form is the most explicit and clear about exactly what changes between the renderers.

[tetrapod00]

Looks like this runs into the same problem as #97646 (comment) - the preprocessor is not available in BaseMaterial3D. Either that's an engine bug that can be fixed, or this PR is not actually the right approach, and something more like #89966 is right.

Set proximity_fade_enabled
  :46 - Tokenizer: Unknown character #35: '#'
  Shader compilation failed.
--Main Shader--
    1 | // NOTE: Shader automatically converted from Godot Engine 4.4.dev's StandardMaterial3D.
...
   44 | 	// Proximity Fade: Enabled
   45 | 	float proximity_depth_tex = textureLod(depth_texture, SCREEN_UV, 0.0).r;
E  46-> 	#if CURRENT_RENDERER == RENDERER_COMPATIBILITY
   47 | 	vec4 proximity_view_pos = INV_PROJECTION_MATRIX * vec4(SCREEN_UV * 2.0 - 1.0, proximity_depth_tex * 2.0 - 1.0, 1.0);
   48 | 	#else
   49 | 	vec4 proximity_view_pos = INV_PROJECTION_MATRIX * vec4(SCREEN_UV * 2.0 - 1.0, proximity_depth_tex, 1.0);
   50 | 	#endif
   51 | 	proximity_view_pos.xyz /= proximity_view_pos.w;
   52 | 	ALPHA *= clamp(1.0 - smoothstep(proximity_view_pos.z + proximity_fade_distance, proximity_view_pos.z, VERTEX.z), 0.0, 1.0);
   53 | }
   54 | 

[akien-mga]

Instead of using the preprocessor, you can do the if/else in C++ and add code to the generated string conditionally.

[tetrapod00]

Instead of using the preprocessor, you can do the if/else in C++ and add code to the generated string conditionally.

Yeah, the hope was to avoid that, so that the material is correct in multiple renderers even after being converted to a ShaderMaterial. The superseded PR already takes a similar approach with conditionally added C++, and was apparently deferred for a while in the hope that we could use the preprocessor somehow instead.