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