freetype/example/round/main.go

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// Copyright 2010 The Freetype-Go Authors. All rights reserved.
// Use of this source code is governed by your choice of either the
// FreeType License or the GNU General Public License version 2 (or
// any later version), both of which can be found in the LICENSE file.
// This program visualizes the quadratic approximation to the circle, used to
// implement round joins when stroking paths. The approximation is used in the
// stroking code for arcs between 0 and 45 degrees, but is visualized here
// between 0 and 90 degrees. The discrepancy between the approximation and the
// true circle is clearly visible at angles above 65 degrees.
package main
import (
"bufio"
"fmt"
"image"
"image/png"
"log"
"math"
"os"
"freetype-go.googlecode.com/hg/freetype/raster"
)
func main() {
const (
n = 17
r = 256 * 80
)
s := raster.Fix32(r * math.Sqrt(2) / 2)
t := raster.Fix32(r * math.Tan(math.Pi/8))
m := image.NewRGBA(800, 600)
for y := 0; y < m.Height(); y++ {
for x := 0; x < m.Width(); x++ {
m.Pixel[y][x] = image.RGBAColor{63, 63, 63, 255}
}
}
mp := raster.NewRGBAPainter(m)
mp.SetColor(image.Black)
z := raster.NewRasterizer(800, 600)
for i := 0; i < n; i++ {
cx := raster.Fix32(25600 + 51200*(i%4))
cy := raster.Fix32(2560 + 32000*(i/4))
c := raster.Point{cx, cy}
theta := math.Pi * (0.5 + 0.5*float64(i)/(n-1))
dx := raster.Fix32(r * math.Cos(theta))
dy := raster.Fix32(r * math.Sin(theta))
d := raster.Point{dx, dy}
// Draw a quarter-circle approximated by two quadratic segments,
// with each segment spanning 45 degrees.
z.Start(c)
z.Add1(c.Add(raster.Point{r, 0}))
z.Add2(c.Add(raster.Point{r, t}), c.Add(raster.Point{s, s}))
z.Add2(c.Add(raster.Point{t, r}), c.Add(raster.Point{0, r}))
// Add another quadratic segment whose angle ranges between 0 and 90 degrees.
// For an explanation of the magic constants 22, 150, 181 and 256, read the
// comments in the freetype/raster package.
dot := 256 * d.Dot(raster.Point{0, r}) / (r * r)
multiple := raster.Fix32(150 - 22*(dot-181)/(256-181))
z.Add2(c.Add(raster.Point{dx, r + dy}.Mul(multiple)), c.Add(d))
// Close the curve.
z.Add1(c)
}
z.Rasterize(mp)
for i := 0; i < n; i++ {
cx := raster.Fix32(25600 + 51200*(i%4))
cy := raster.Fix32(2560 + 32000*(i/4))
for j := 0; j < n; j++ {
theta := math.Pi * float64(j) / (n - 1)
dx := raster.Fix32(r * math.Cos(theta))
dy := raster.Fix32(r * math.Sin(theta))
m.Set(int((cx+dx)/256), int((cy+dy)/256), image.Yellow)
}
}
// Save that RGBA image to disk.
f, err := os.Open("out.png", os.O_CREAT|os.O_WRONLY, 0600)
if err != nil {
log.Stderr(err)
os.Exit(1)
}
defer f.Close()
b := bufio.NewWriter(f)
err = png.Encode(b, m)
if err != nil {
log.Stderr(err)
os.Exit(1)
}
err = b.Flush()
if err != nil {
log.Stderr(err)
os.Exit(1)
}
fmt.Println("Wrote out.png OK.")
}