1.将矩形旋转45度(PI/4)

#define PI 3.1415926

float Box(vec2 uv,vec2 bottomLeft,vec2 topRight,vec2 blur)
{
    vec2 c = smoothstep(bottomLeft,bottomLeft + blur,uv);
    vec2 co2 = smoothstep(topRight,topRight - blur ,uv);
    c *= co2;
    return c.x * c.y;
}


mat2 rotate2d(float angle)
{
    mat2 m = mat2(cos(angle),-sin(angle),sin(angle),cos(angle));
    return m;
}

void mainImage( out vec4 fragColor, in vec2 fragCoord )
{
    // Normalized pixel coordinates (from 0 to 1)
    vec2 uv = fragCoord/iResolution.xy;
    uv.x *= iResolution.x / iResolution.y;
    uv *= 4.0;
    uv = fract(uv);

    uv = uv * rotate2d(PI / 4.0);

    float c1 = Box(uv,vec2(-0.0,-0.0),vec2(1.5,1.5),vec2(0.01,0.01));

    fragColor = vec4(c1 * vec3(1.0,1.0,0.0),1.0);
}

效果:

2.分段处理

#define PI 3.1415926

float Box(vec2 uv,vec2 bottomLeft,vec2 topRight,vec2 blur)
{
    vec2 c = smoothstep(bottomLeft,bottomLeft + blur,uv);
    vec2 co2 = smoothstep(topRight,topRight - blur ,uv);
    c *= co2;
    return c.x * c.y;
}


mat2 rotate2d(float angle)
{
    mat2 m = mat2(cos(angle),-sin(angle),sin(angle),cos(angle));
    return m;
}

void mainImage( out vec4 fragColor, in vec2 fragCoord )
{
    // Normalized pixel coordinates (from 0 to 1)
    vec2 uv = fragCoord/iResolution.xy;
    uv.x *= iResolution.x / iResolution.y;
    uv *= 4.0;
    float id = mod(uv.x,2.0);
    float idy = mod(uv.y,2.0);
    int idd = int(floor(id));
    int iddy = int(floor(idy));
    uv = fract(uv);
    if(iddy == 0){
        //左右翻转
        uv.x = 1.0 - uv.x;
    }
    if(idd == 0){
        uv.y = 1.0 - uv.y;
        uv = uv * rotate2d(PI / 4.0);
    }else if(idd == 1){
        uv = uv * rotate2d(PI / 4.0);
    }else if(idd == 2){
        uv.x = 1.0 - uv.x;
        uv = uv * rotate2d(-PI / 4.0);
    }


    float c1 = Box(uv,vec2(-0.0,-0.0),vec2(1.5,1.5),vec2(0.01,0.01));

    fragColor = vec4(c1 * vec3(1.0,1.0,0.0),1.0);
}

效果: