Update force field shader

[tetrapod00]

3.5: 4.3 Compatibility: 4.3 Forward+:

Changes:

• Remove ball throwing from demo. This was pretty bad, as you couldnt see the shader in the editor, and using the demo required a hidden action.
• Split out the material so it's visible in the filesystem.
• Renamed many parameters to be more accurate.
• Add hint range sliders
• Add inspector headers. This one I'm not sure about. I don't like how they're folded by default, but that is a godot editor setting you can change. Feel free to suggest renames for these, too.
• Add inspector comments. I added comments to every parameter on this one, but that means there are some stub comments. The way inspector comments work right now, I don't know if you can add newlines in the shader without adding a paragraph break in the inspector description. This means some of the comments are very wide when looking at the shader itself.
• Change linear depth calculation to the 4.3 one, and add code which works for both forward+ and compatibility.

Sliders

Linear depth:

The new linear depth calculation looks like this:

float get_linear_depth(sampler2D depth_texture, vec2 screen_uv, mat4 inv_projection_matrix, bool use_opengl_ndc) {
    float depth_raw = texture(depth_texture, screen_uv).x;
    vec3 ndc;
    if (use_opengl_ndc){
        // Compatibility. Uses OpenGL NDC space: range of [-1,1] for NDC.xy, range of [-1,1] for NDC.z
        ndc = vec3(screen_uv, depth_raw) * 2.0 - 1.0;
    } else {
        // Forward+ or Mobile. Uses Vulkan NDC space: range of [-1,1] for NDC.xy, range of [0,1] for NDC.z
        ndc = vec3(screen_uv * 2.0 - 1.0, depth_raw); 
    }
    vec4 position_view = inv_projection_matrix * vec4(ndc, 1.0);
    position_view.xyz /= position_view.w;
    return -position_view.z;
}

Call site:

    // Get the linear depth.
    float depth = get_linear_depth(DEPTH_TEXTURE, SCREEN_UV, INV_PROJECTION_MATRIX, OUTPUT_IS_SRGB);

This is the same one used by the official Advanced Postprocessing tutorial, and the official VisualShader node LinearSceneDepth. It may be more expensive than the 3.x version. To be honest, I don't fully understand the 3.x version. Relying on the internals of the matrices, like the 3.x version was doing, may be wrong for code like this that also functions partially as a tutorial. I may come back and see if there's a simpler or more efficient way to get the linear depth in 4.3, but as is this functions.

OUTPUT_IS_SRGB is a proxy for whether the renderer is Compatibility or Forward+/Mobile. I confirmed with clayjohn that it for this purpose it won't change until 5.0. Ideally this would be an explicit #define for the renderer, but that's not actually currently available. I am a little wary of putting a snippet like this, with the branching, into code that will be copied and pasted a lot. But as is, code that is copied and pasted just assumes Vulkan NDC space, which is incorrect.

We need to check the renderer because Compatibility and Forward+/Mobile use different NDC spaces.

Compatibility, if we don't check, and assume Vulkan Forward+ NDC:

Compatibility if we check:

[NathanLovato]

Excellent work, thank you so much for going the extra mile! The experience using the shader is great. Also, thank you for all the details.

For next PRs, please note that you don't have to bother taking many screenshots. But, I always test directly in Godot, take the time to read the code, and it's already very clear and even commented so it already makes reviewing very easy. So, you don't have to use more of your time to present things neatly on GitHub. I really appreciate your thoughtfulness and attention to detail.