thebookofshaders/14/texture-sprite.frag

#ifdef GL_ES
precision mediump float;
#endif

uniform sampler2D u_tex0;
uniform vec2 u_tex0Resolution;

int col = 5;
int row = 4;

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(0.0);

    // Resolution of one frame
    vec2 fRes = u_tex0Resolution/vec2(float(col),float(row));

    // Normalize value of the frame resolution 
    vec2 nRes = u_tex0Resolution/fRes;

    // Scale the coordenates to a single frame
    st = st/nRes;

    // Calculate the offset in cols and rows
    float timeX = u_time*15.; 
    float timeY = floor(timeX/float(col));
    vec2 offset = vec2( floor(timeX)/nRes.x, 
                        1.0-(floor(timeY)/nRes.y) );
    st = fract(st+offset);

    color = texture2D(u_tex0,st);

    gl_FragColor = color;
}