Scrolling Texture Rotation With Fragment And Vertex Shader Gets Faster

NNorthProse11 · Dec 21, 2020

I have the following canvas item shader code. The problem with it is that rotation seems to speed up over time. I suspect this is caused by me also offsetting the UV over time. Im looking for way around this. Here is my code:

shader_type canvas_item;

uniform float rotation;
uniform float speed_scale;

vec2 rotateUV(vec2 uv, vec2 pivot, float rot) {
    float cosa = cos(rot);
    float sina = sin(rot);
    uv -= pivot;
    return vec2(
        cosa * uv.x - sina * uv.y,
        cosa * uv.y + sina * uv.x 
    ) + pivot;
}

void vertex() {
    UV.y += time * speed_scale;

}

void fragment(){
    vec4 color;
    color = texture(TEXTURE, rotateUV(UV, vec2(0.5), rotation ));
    COLOR = color;
}

Megalomaniak · Dec 21, 2020

I'm not sure you want to use sine waves(sin & cos) if what you are looking for is linear interpolation. Sine waves are essentially bell curves so the "acceleration" you see is part of the curve.

NNorthProse11 · Dec 22, 2020

Hmm, thing is though, if I stop offsetting the UV to create the texture scrolling effect, the rotation speed seems constant. I'm not very math savvy, so that rotateUV function comes from here:

https://godotengine.org/qa/41400/simple-texture-rotation-shader

Megalomaniak · Dec 22, 2020

Hm, well depending on how you use it a sine wave can also give you a linear cycle. For a rotation it might work out that way, for translation it can easily end up a speed up to peak to slow down sort of non-linear interpolation. It's been a while since I've toyed with shaders and sinewaves tho.