raysan5.raylib/examples/resources/shaders/grayscale.fs

#version 330

in vec2 fragTexCoord;

out vec4 fragColor;

uniform sampler2D texture0;
uniform vec4 tintColor;

// NOTE: Add here your custom variables

void main()
{
    vec4 base = texture2D(texture0, fragTexCoord)*tintColor;
    
    // Convert to grayscale using NTSC conversion weights
    float gray = dot(base.rgb, vec3(0.299, 0.587, 0.114));
    
    fragColor = vec4(gray, gray, gray, tintColor.a);
}