Graphics Reference
In-Depth Information
Example 14-21
Anti-Aliased Checker Fragment Shader
(continued)
void main()
{
vec4 color;
vec4 color0 = vec4(0.0);
vec4 color1 = vec4(1.0);
vec2 st_width;
vec2 fuzz;
vec2 check_pos;
float fuzz_max;
// calculate the filter width
st_width = fwidth(v_st);
fuzz = st_width * float(u_frequency) * 2.0;
fuzz_max = max(fuzz.s, fuzz.t);
// get the place in the pattern where we are sampling
check_pos = fract(v_st * float(u_frequency));
if (fuzz_max <= 0.5)
{
// if the filter width is small enough, compute
// the pattern color by performing a smooth interpolation
// between the computed color and the average color
vec2 p = smoothstep(vec2(0.5), fuzz + vec2(0.5),
check_pos) + (1.0 − smoothstep(vec2(0.0), fuzz,
check_pos));
color = mix(color0, color1,
p.x * p.y + (1.0 − p.x) * (1.0 − p.y));
color = mix(color, (color0 + color1)/2.0,
smoothstep(0.125, 0.5, fuzz_max));
}
else
{
// filter is too wide; just use the average color
color = (color0 + color1)/2.0;
}
outColor = color;
}
Figure 14-13 shows the checkerboard image rendered using the anti-
aliased fragment shader in Example 14-18 with
u_frequency
= 10.
To anti-alias the checkerboard procedural texture, we need to estimate
the average value of the texture over an area covered by the pixel. Given
a function
g(v)
that represents a procedural texture, we need to calculate
