Batch drawing 2D circles while having different opacity for some

[dusty karma]

Hi,
my game is 2D and has a bunch of circles of the same color and mesh, so I have done something similar to this (But in 2D).

However, my use case requires the alteration of the opacity of individual dots based on a distance to the player. As stated, I have many circles and wish to do a batched draw call for all the circles instead of individual ones, since they all share the same mesh and the same material, except the alpha.

I would like to use an automated method if possible, and I'd be open to trying a custom shader, but I have no experience with them in bevy / wgsl.

[dark crescent]

So, each circle is a dot, yes?

If as you say, you want to keep the same material for all the dots then a custom shader is what you want. In the material and shader, you'll add a position for your player, then in the shader you calculate the distance and then your alpha. You'll have to update the material each frame to update the player position.

[dusty karma]

Out of curiosity, how many of those dots doc you think I should have to implement this and not count as premature optimization?

[scarlet cypress]

You can do several thousand just as entities no problem

[dusty karma]

I've tried that method and I failed to get the shader to compile. Here is what I have:

#import bevy_pbr::forward_io::VertexOutput

@group(0) @binding(0) var<uniform> player_position: vec2<f32>;
@group(0) @binding(1) var<uniform> fade_distance: f32;
@group(0) @binding(2) var<uniform> min_opacity: f32;
@group(0) @binding(3) var<uniform> base_color: vec4<f32>;

@fragment
fn fs(input: VertexOutput) -> @location(0) vec4<f32> {
    let dist = distance(input.world_position, player_position);
    let alpha = max(min_opacity, 1.0 - (dist / fade_distance));
    return vec4(base_color.rgb, alpha * base_color.a);
}

When running, the shader doesn't compile and says that:

 2025-01-31T15:03:02.080804Z ERROR bevy_render::render_resource::pipeline_cache: failed to process shader:
 error: failed to build a valid final module: Entry point fs at Fragment is invalid
    ┌─ shaders/particle_shader.wgsl:10:16
    │
 10 │     let dist = distance(input.world_position, player_position);
    │                ^^^^^^^^ naga::Expression [4]
    │
    = Expression [4] is invalid
    = Argument [1] to Distance as expression [3] has an invalid type.

[dusty karma]

ok I've solved it

input.world_position should be changed to input.world_position.xy, because my player position is 2d

also I had to change the alpha mode to blend or it wouldn't accually do anything