Drawing a rectangle in a Fragment Shader

Question

I'd like to know if there is a possibility to draw a rectangle in a fragment shader without using any if. I tried this :

void main(void)
{ 

    vec2 resolution = vec2( 300.0, 300.0 );
    vec2 center = resolution / 2.0;
    vec2 position = ( gl_FragCoord.xy - center ) / resolution;

    if ( ( abs(position.x) < 0.5 ) && ( abs(position.y) < 0.5 ) )
        gl_FragColor = vec4( 0.0, 0.0, 1.0, 1.0 );
}

And when i test if the pixel is in the rectangle i must perform an if test. I think there's a solution not to have an if, if you can help me ?

Answer 1

I'm not sure what you are trying to do but here's one idea?

precision mediump float;

uniform float time;
uniform vec2 mouse; 
uniform vec2 resolution;

void main( void ) {

    vec2 position = ( gl_FragCoord.xy / resolution.xy ) + mouse / 4.0;

                   // x1  y1   x2   y2
    vec4 rect = vec4(0.2, 0.3, 0.4, 0.5);
    vec2 hv = step(rect.xy, position) * step(position, rect.zw);
    float onOff = hv.x * hv.y;

    gl_FragColor = mix(vec4(0,0,0,0), vec4(1,0,0,0), onOff);
}

Here's a working version