Fix for multimesh motion vector corruption

[Lssikkes]

Multimeshes can get corrupted when using multimesh_set_buffer, and subsequently using the individual set API afterwards. This commit fixes the issue by making sure motion vectors are always disabled after resetting the instance count, and filling both halves of the buffer on initial multimesh_set_buffer motion vector upgrade.

[DarioSamo]

Change looks good to me and makes sense. I haven't personally tested it but I discussed the change with @Lssikkes and we agreed this seemed like a good way to fix the issue.

I'll wait for someone else to verify and make sure it doesn't cause regressions. This could make it either for 4.3 or wait to be cherry-picked for 4.3.1.

[Lssikkes]

Thank you AThousandShips & Dario. Made the style changes.

[ciso507]

This is going on on godot 4.3 stable Compiled version"
i had an issue with c++ multimesh, it doesnt update the visuals of motion bullets.. but it keeps updating the position.
Im also using c++ multimeshes for decorations ... static enviromental elements>> it updates the visuals and also the position.

[akien-mga]

@ciso507 This PR hasn't been merged yet, so it's expected that you might still see this bug in 4.3.stable.
It will be merged soon (like in minutes, I'm working on a first merge batch) for 4.4, and cherry-picked for 4.3.1.

[akien-mga]

Thanks! And congrats for your first merged Godot contribution 🎉

[ciso507]

@ciso507 This PR hasn't been merged yet, so it's expected that you might still see this bug in 4.3.stable. It will be merged soon (like in minutes, I'm working on a first merge batch) for 4.4, and cherry-picked for 4.3.1.

Case#1:
I tried to reproduce on a brand new project and the bullets visuals will show if i dont animate the sprite trough the shader, if its just without animation it will show . It happens when i turn on HDR. without HDR it animates and show each individual mesh from the multimesh2d without issues.

Case#2: (I created a new project for this and i couldn't reproduce it)
And there is this error for normal instantiate of a bullet scene : error "" Failed to instantiate scene state of "", node count is 0. Make sure the PackedScene resource is valid. "" I also noted that my biggest scene loaded like 30% faster nice job 👍

[Lssikkes]

Thanks! And congrats for your first merged Godot contribution 🎉

Awesome, thank you so much :)

I think there are some issues remaining, I saw one case where just custom data/color updates could still cause confusion with the buffers, and the colors would start to flutter between state 1 and 2. I think I'll make a test scene soon to try to test all these different permutations of being able to use MultiMesh, and see which combination of things are still causing issues. This MR already improves things by quite a bit though.

Case#1:
I tried to reproduce on a brand new project and the bullets visuals will show if i dont animate the sprite trough the shader, if > its just without animation it will show . It happens when i turn on HDR. without HDR it animates and show each individual ?
mesh from the multimesh2d without issues.

This might be a separate issue - is 2D using motion vectors? Fullscreen motion blur?

[Lssikkes]

(the road is rendered through custom data with a shader interpreting them as texture coordinates)

[ciso507]

@Lssikkes @akien-mga
Thanks for the reply. I haven't interacted with motion blur for 2d; i dont know if its triggered by default (where can i check?) >> this shader works without hdr it will animate the texture if the multimesh instance2d that has a bool is enabled [should animate: true] now if i turn the enabled while hdr is enabled it will not display the texture or the animation... and the meshinstance2d shows for 0.2seconds and then it dissapear.
The shader im using :

<
shader_type canvas_item;

uniform float col;
uniform float row;
uniform vec4 custom;
uniform vec4 glow_color;
uniform float glow_intensity : hint_range(0.1, 47.0); // New uniform for glow intensity

void fragment() {
    if (custom.r == 1.0) {
        vec2 uv = UV;

        if (col != 0.0 || row != 0.0) {
            uv = vec2(uv.x / row, uv.y / col);
            uv.y += custom.b / col;
            uv.x += custom.g / row;
        }

        // Sample the texture
        vec4 texture_color = texture(TEXTURE, uv);
        
        // Only apply glow if the texture has opacity
        if (texture_color.a > 0.0) {
            // Apply glow to the texture, ensuring it is masked by the alpha channel
            vec4 glow = texture(TEXTURE, uv) * glow_color * glow_intensity * texture_color.a;

            // Mix the original texture and glow based on the alpha channel
            COLOR = mix(texture_color, glow, texture_color.a);
        } else {
            COLOR = texture_color;
        }
    } else {
        COLOR = vec4(0.0, 0.0, 0.0, 0.0);
    }
}
>

[ciso507]

@akien-mga hey it seems that it gets fixed if i change the timer.wait_time ... that sends the signal for the width and height to change the frames, well this doesnt make sense to me that it gets fixed like this but its fixed.

[akien-mga]

Cherry-picked for 4.3.1.