thebookofshaders/15/texture.frag

// Author @patriciogv - 2015
// http://patriciogonzalezvivo.com

#ifdef GL_ES
precision mediump float;
#endif

uniform sampler2D u_tex0;
uniform vec2 u_tex0Resolution;

uniform vec2 u_resolution;
uniform vec2 u_mouse;
uniform float u_time;

void main () {
    vec2 st = gl_FragCoord.xy/u_resolution.xy;
    vec4 color = vec4(st.x,st.y,0.0,1.0);

    color = texture2D(u_tex0,st);

    gl_FragColor = color;
}