Add support for soft shadows to the GPU lightmapper

[techiepriyansh]

master port of #50184.
Closes #52199.

Results

Directional Light

• w/o soft shadows
  
• w/ soft shadows (size = 10deg)
  

Spot Light

• w/o soft shadows
  

• w/ soft shadows (size = 1m)
  

Omni Light

• w/o soft shadows
  

• w/ soft shadows (size = 1m)
  

[jcostello]

Nice result

[Calinou]

I tested this PR locally, it's looking good 🙂

The PCSS shadow blur between real-time and baked shadows seems to be a close match in the default configuration.

The only issue I could notice is that the Light3D's Shadow Blur property isn't taken into account to increase/decrease the shadow blur (even after baking lightmaps again). Also, constant shadow blur (when size = 0.0 and blur > 0.0) isn't supported, but I suppose this is outside the scope of this PR.

I suggest multiplying the shadow blur in the lightmapper by the Shadow Blur property. This would allow for a better matching between real-time and baked shadows when Shadow Blur is modified from its default value (1.0).

Testing project: test_lightmap_shadow_blur.zip

Shadow Blur = 1.0

Shadow Blur = 2.0

[techiepriyansh]

Updated the PR to respect the Shadow Blur property. Needs testing.

[jcostello]

Angular Distance and blur taken into consideration. Althought it might have the same result because at edges the shadow is not blur at all. So distance and blur affect each other as a multiply. If Angular Distance is 0, blur has no effect.

Angular Distance 2, Blur 4

Angular Distance 0, Blur 4

[clayjohn]

Angular Distance and blur taken into consideration. Althought it might have the same result because at edges the shadow is not blur at all. So distance and blur affect each other as a multiply. If Angular Distance is 0, blur has no effect.

In the regular scene shader, we still take the soft_shadow_scale into account to scale PCF shadows
For Omnilight when light size == 0

godot/servers/rendering/renderer_rd/shaders/scene_forward_lights_inc.glsl

Lines 503 to 517 in f3bf033

	} else {
	vec4 uv_rect = base_uv_rect;
	vec3 shadow_sample = normalize(shadow_dir + normal_bias);
	if (shadow_sample.z >= 0.0) {
	uv_rect.xy += flip_offset;
	flip_offset *= -1.0;
	}
	shadow_sample.z = 1.0 + abs(shadow_sample.z);
	vec2 pos = shadow_sample.xy / shadow_sample.z;
	float depth = shadow_len - omni_lights.data[idx].shadow_bias;
	depth *= omni_lights.data[idx].inv_radius;
	shadow = sample_omni_pcf_shadow(shadow_atlas, omni_lights.data[idx].soft_shadow_scale / shadow_sample.z, pos, uv_rect, flip_offset, depth);
	}

For directional light when angular diameter == 0

godot/servers/rendering/renderer_rd/shaders/scene_forward_clustered.glsl

Line 1545 in f3bf033

shadow = sample_directional_pcf_shadow(directional_shadow_atlas, scene_data.directional_shadow_pixel_size * directional_lights.data[i].soft_shadow_scale * blur_factor, pssm_coord);

Overall, I'm not sure we need to have blur when Angular Distance is 0. Blur is present in the default PCF path for shadows in order to hide pixelation artifacts from using low resolution shadowmaps. For lightmaps, we can get proper hard shadows that look really nice, so the blurring is less beneficial

[Calinou]

Blur is present in the default PCF path for shadows in order to hide pixelation artifacts from using low resolution shadowmaps. For lightmaps, we can get proper hard shadows that look really nice, so the blurring is less beneficial

This is only true if bicubic sampling is used, which is not the case in master yet. You can't always use high-resolution lightmaps for storage and bake time reasons, so it's important that low-resolution lightmaps look good too.

That said, I agree that contact hardening (PCSS) is preferable to constant blur when using fully baked lights.

[clayjohn]

Looks great! Thank you for your hard work

[akien-mga]

Thanks!