110 lines
3.4 KiB
Go
110 lines
3.4 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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// +build example
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//
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// This build tag means that "go install github.com/tdewolff/freetype/..."
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// doesn't install this example program. Use "go run main.go" to run it or "go
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// install -tags=example" to install it.
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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/color"
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"image/draw"
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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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"github.com/tdewolff/freetype/raster"
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"golang.org/x/image/math/fixed"
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)
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// pDot returns the dot product p·q.
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func pDot(p, q fixed.Point26_6) fixed.Int52_12 {
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px, py := int64(p.X), int64(p.Y)
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qx, qy := int64(q.X), int64(q.Y)
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return fixed.Int52_12(px*qx + py*qy)
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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 = 64 * 80
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)
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s := fixed.Int26_6(r * math.Sqrt(2) / 2)
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t := fixed.Int26_6(r * math.Tan(math.Pi/8))
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m := image.NewRGBA(image.Rect(0, 0, 800, 600))
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draw.Draw(m, m.Bounds(), image.NewUniform(color.RGBA{63, 63, 63, 255}), image.ZP, draw.Src)
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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 := fixed.Int26_6(6400 + 12800*(i%4))
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cy := fixed.Int26_6(640 + 8000*(i/4))
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c := fixed.Point26_6{X: cx, Y: cy}
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theta := math.Pi * (0.5 + 0.5*float64(i)/(n-1))
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dx := fixed.Int26_6(r * math.Cos(theta))
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dy := fixed.Int26_6(r * math.Sin(theta))
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d := fixed.Point26_6{X: dx, Y: 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(fixed.Point26_6{X: r, Y: 0}))
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z.Add2(c.Add(fixed.Point26_6{X: r, Y: t}), c.Add(fixed.Point26_6{X: s, Y: s}))
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z.Add2(c.Add(fixed.Point26_6{X: t, Y: r}), c.Add(fixed.Point26_6{X: 0, Y: r}))
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// Add another quadratic segment whose angle ranges between 0 and 90
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// degrees. For an explanation of the magic constants 128, 150, 181 and
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// 256, read the comments in the freetype/raster package.
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dot := 256 * pDot(d, fixed.Point26_6{X: 0, Y: r}) / (r * r)
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multiple := fixed.Int26_6(150-(150-128)*(dot-181)/(256-181)) >> 2
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z.Add2(c.Add(fixed.Point26_6{X: dx, Y: 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 := fixed.Int26_6(6400 + 12800*(i%4))
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cy := fixed.Int26_6(640 + 8000*(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 := fixed.Int26_6(r * math.Cos(theta))
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dy := fixed.Int26_6(r * math.Sin(theta))
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m.Set(int((cx+dx)/64), int((cy+dy)/64), color.RGBA{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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outFile, err := os.Create("out.png")
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if err != nil {
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log.Println(err)
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os.Exit(1)
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}
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defer outFile.Close()
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b := bufio.NewWriter(outFile)
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err = png.Encode(b, m)
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if err != nil {
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log.Println(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.Println(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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