smoother transition

[24bit192kHz]

Hi
is there a way I make it smoother? like I don't want it to just jump to %80 brightness I want to go from %100 to %99 etc with sleep of 0.5s for example

my current shader

precision mediump float;
varying vec2 v_texcoord;
uniform sampler2D tex;

void main() {
    vec4 pixColor = texture2D(tex, v_texcoord);

    // Red
    pixColor[0] *= 0.800;

    // Green
    pixColor[1] *= 0.800;

    // Blue
    pixColor[2] *= 0.800;

    gl_FragColor = pixColor;
}