iOS GLSL.有没有办法使用GLSL着色器创建图像直方图？

#version 300 es
precision highp float;

uniform sampler2D inputImageTexture;
uniform highp float height;
uniform highp float width;
out vec4 FragColor;

void main() {
    highp float xCoord = gl_FragCoord.x;
    highp float yCoord = gl_FragCoord.y;
    highp float red = 0.0;
    highp float green = 0.0;
    highp float blue = 0.0;

    for (int i = 0; i < int(width); i++) {
        for (int j = 0; j < int(height); j++) {
            vec4 texel = texture(inputImageTexture, vec2(float(i) / width, float(j) / height));
            red += texel.r;
            green += texel.g;
            blue += texel.b;
        }
    }

    red /= (width * height);
    green /= (width * height);
    blue /= (width * height);

    FragColor = vec4(red, green, blue, 1.0);
}