Read GDShader variable value from a GDScript

Question

Im learning shaders in Godot, and I need to read the value of a variable from a shader. I currently have a similar setup to the following:

shader_type canvas_item;

varying float value;

void fragment()
{
    value = some_data;
}

Now, having a GDScript attached to the same Node, how can I read value? Ive tried using custom Viewports and ComputeBuffers, but I cant get them to work.

What I am trying to do is to create a kind of like "light" shader, so that the alpha from each pixel decreases as the mouse gets further from that pixel. However, I wasnt able to get it to work, and so I was trying to print said distance from a GDScript, and that lead to this question. The following is my current attempt to that shader:

shader_type canvas_item;

uniform vec2 mousePos; //This variable is filled from another GDScript
uniform float lightRadius = 10.0;

void fragment() {
    // Called for every pixel the material is visible on.
    //COLOR = vec4(sin(UV.x), cos(UV.y), 0.0, 1.0);
    COLOR = texture(TEXTURE, UV);
    float dist = distance(mousePos, FRAGCOORD.xy);
    COLOR.a = dist;
}

Since I couldnt get it to work, I was trying to print dist from another script.

Edit: explanation of my problem. Thanks in advance!

This sounds like a XY Problem. What are you trying to do? Reading from the GPU is expensive; currently, I would suggest to reconstruct value from the same logic of some_data in your script. If you elaborate more and provide details, we will be able to give a more precise solution to your issue. — liggiorgio
– liggiorgio, Commented Apr 25 at 20:36

florianvazelle · Accepted Answer · 2025-06-16 12:57:41Z

0

Depending on the type of your node, you may have a method for recovering the current material applied to it. Please refer to the godot documentation: https://docs.godotengine.org/.

This varies from 3d or 2d, it can be get_material_overlay or get_material_override ...

So, in your gdscript, you can get the material with (for example) get_material_overlay. And you can read the shader value with get_shader_parameter.

This gives :

func _ready():
    print(get_material_overlay().get_shader_parameter("dist"))

And your shader :

shader_type canvas_item;

uniform float dist;

uniform vec2 mousePos; //This variable is filled from another GDScript
uniform float lightRadius = 10.0;

void fragment() {
    // Called for every pixel the material is visible on.
    //COLOR = vec4(sin(UV.x), cos(UV.y), 0.0, 1.0);
    COLOR = texture(TEXTURE, UV);
    dist = distance(mousePos, FRAGCOORD.xy);
    COLOR.a = dist;
}

edited Jun 16 at 12:57

answered Jun 16 at 12:42

florianvazelle

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