坐标操作

• out , in
• vec2, vec3, vec4
• fragColor, fragCoord
• iResolution

简单的颜色变化

void mainImage (out vec4 fragColor, in vec2 fragCoord){
    vec2 uv = fragCoord / iResolution.xy;
    fragColor = vec4(uv, 0, 1.); // rgba
}

画个圆

void mainImage (out vec4 fragColor, in vec2 fragCoord){
    vec2 uv = fragCoord / iResolution.xy - vec2(.5);
    float c = 0.0;
    float r = 0.3;
    if(length(uv) < r){
      c = 1.0;
    }
    fragColor = vec4(vec3(c),1.0); // rgba
}

画个正圆

void mainImage (out vec4 fragColor, in vec2 fragCoord){
    vec2 uv = (fragCoord - 0.5 * iResolution.xy)/min(iResolution.x,iResolution.y);
    float c = 0.0;
    float r = 0.3;
    if(length(uv) < r){
      c = 1.0;
    }
    fragColor = vec4(vec3(c),1.0); // rgba
}