一个圆

void mainImage( out vec4 fragColor, in vec2 fragCoord )
{
    // Normalized pixel coordinates (from 0 to 1)
    vec2 uv = fragCoord/iResolution.xy;

    uv -= 0.5;// -.5 <> .5
    uv.x *= iResolution.x / iResolution.y;
    float d = length(uv);
    float r = 0.3;
    float c = smoothstep(r , r -0.05, d);
    //if ( d < 0.3) d = 1.; else d = 0.;

    // Output to screen
    fragColor = vec4(vec3(c),1.0);
}

微笑小黄脸

float Circle (vec2 uv, vec2 p, float r, float blur)
{
    float d = length(uv - p);
    float c = smoothstep(r , r - blur, d);
    return c;
}
void mainImage( out vec4 fragColor, in vec2 fragCoord )
{
    // Normalized pixel coordinates (from 0 to 1)
    vec2 uv = fragCoord/iResolution.xy;

    uv -= 0.5;// uv -= vec2(.5, .5);
    uv.x *= iResolution.x / iResolution.y;
    //颜色
    vec3 col = vec3(0);
    //眼睛
    float c = Circle(uv, vec2(0), .4, .05);
    c -= Circle(uv, vec2(.1, .1), .07, .01);
    c -= Circle(uv, vec2(-.1, .1), .07, .01);
    //嘴巴
    float mouth = Circle(uv, vec2(0.,0.), .25, .01);
    mouth -= Circle(uv, vec2(0., 0.05), .25, .01);
    c -= mouth;

    // Output to screen
    col = vec3(1., 1., 0.)*c;
    
    fragColor = vec4(col,1.0);
}

float Circle (vec2 uv, vec2 p, float r, float blur)
{
    float d = length(uv - p);
    float c = smoothstep(r , r - blur, d);
    return c;
}
float smiley(vec2 uv, vec2 p, float size){
    uv -= p;//转换坐标系
    uv /= size;//缩放坐标系
    
    float c = Circle(uv, vec2(0), .4, .01);//c就是mask遮罩
    c -= Circle(uv, vec2(.1, .1), .07, .01);
    c -= Circle(uv, vec2(-.1, .1), .07, .01);
    
    float mouth = Circle(uv, vec2(0.,0.), .25, .01);
    mouth -= Circle(uv, vec2(0., 0.05), .25, .01);
    c -= mouth;
    return c;
}
void mainImage( out vec4 fragColor, in vec2 fragCoord )
{
    // Normalized pixel coordinates (from 0 to 1)
    vec2 uv = fragCoord/iResolution.xy;

    uv -= 0.5;// uv -= vec2(.5, .5);
    uv.x *= iResolution.x / iResolution.y;
    
    vec3 col = vec3(0);
    
    float mask = smiley(uv, vec2(0,0.), 1.);
    //col = vec3(mouth);
    //if ( d < 0.3) d = 1.; else d = 0.;
    //vec3 col = 0.5 + vec3(0,2,4);
    // Time varying pixel color
    
    //vec3 col = 0.5 + 0.5*cos(iTime+uv.xyx+vec3(0,2,4));

    // Output to screen
    //fragColor = vec4(col,1.0);
    col = vec3(1., 1., 0.)*mask;
    
    fragColor = vec4(col,1.0);
}

矩形

float Band(float t, float start, float end, float blur)
{
    float step1 = smoothstep(start - blur, start + blur, t);
    float step2 = smoothstep(end + blur, end - blur, t);
    
    return step1 * step2;
}
float Rect(vec2 uv, float left,float right, float bottom, float top, float blur){
    float band1= Band(uv.x, left, right, blur);
    float band2= Band(uv.y, bottom, top, blur);
    
    return band1 * band2;
}
void mainImage( out vec4 fragColor, in vec2 fragCoord )
{
    // Normalized pixel coordinates (from 0 to 1)
    vec2 uv = fragCoord/iResolution.xy;

    uv -= 0.5;// uv -= vec2(.5, .5);
    uv.x *= iResolution.x / iResolution.y;
    
    vec3 col = vec3(0);
    
    float mask = 0.;
    // Output to screen
    mask = Rect(uv, -0.2, .2, -0.2, .2, .01);
    col = vec3(1., 1., 1.)*mask;
    
    fragColor = vec4(col,1.0);
}