[日常] Go语言圣经--浮点数习题

练习 3.1：如果f函数返回的是无限制的float64值，那么SVG文件可能输出无效的多边形元素（虽然许多SVG渲染器会妥善处理这类问题）。修改程序跳过无效的多边形。

练习 3.2：试验math包中其他函数的渲染图形。你是否能输出一个egg box、moguls或a saddle图案?

练习 3.3：根据高度给每个多边形上色，那样峰值部将是红色(#ff0000)，谷部将是蓝色(#0000ff)。

练习 3.4：参考1.7节Lissajous例子的函数，构造一个web服务器，用于计算函数曲面然后返回SVG数据给客户端。服务器必须设置Content-Type头部：

w.Header().Set("Content-Type", "image/svg+xml")
（这一步在Lissajous例子中不是必须的，因为服务器使用标准的PNG图像格式，可以根据前面的512个字节自动输出对应的头部。）允许客户端通过HTTP请求参数设置高度、宽度和颜色等参数。

// Surface computes an SVG rendering of a 3-D surface function.
package main
 
import (
        "fmt"
        "math"
        "net/http"
        "strconv"
)
 
var height,width float64 = 300 , 600 
var     cells         float64= 100                 // number of grid cells
var     xyrange       float64= 30.0                // axis ranges (-xyrange..+xyrange)
var     xyscale       float64= width / 2 / xyrange // pixels per x or y unit
var     zscale        float64= height * 0.4        // pixels per z unit
var     angle         float64= math.Pi / 6         // angle of x, y axes (=30°)
 
 
var sin30, cos30 = math.Sin(angle), math.Cos(angle) // sin(30°), cos(30°)
 
 
func main() {
 
        //http服务
        http.HandleFunc("/",handle)
        http.ListenAndServe("0.0.0.0:8000",nil)
}
func handle(w http.ResponseWriter,r *http.Request){
        w.Header().Set("Content-Type", "image/svg+xml")
        if err := r.ParseForm();err != nil{
                return
        }   
        //var height int
        //var width  int
        for k,v := range r.Form{
                if k == "height"{
                        h, _ := strconv.ParseFloat(v[0],64)
                        if h>0 {height = h}
                }   
                if k == "width"{
                        w, _ := strconv.ParseFloat(v[0],64)
                        if w>0 {width = w}
                }   
        }   
        xyscale = width / 2 / xyrange
        zscale = height * 0.4 
        fmt.Fprintf(w,"<svg xmlns='http://www.w3.org/2000/svg' "+
                "style='stroke: #ff0000; fill: #0000ff; stroke-width: 0.7' "+
                "width='%d' height='%d'>", width, height)
        for i := 0; i < int(cells); i++ {
                for j := 0; j < int(cells); j++ {
                        ax, ay := corner(i+1, j)
                        bx, by := corner(i, j)
                        cx, cy := corner(i, j+1)
                        dx, dy := corner(i+1, j+1)
                        fmt.Fprintf(w,"<polygon points='%g,%g %g,%g %g,%g %g,%g'/>\n",
                                ax, ay, bx, by, cx, cy, dx, dy)
                }
        }
        fmt.Fprintf(w,"</svg>")
 
}
func corner(i, j int) (float64, float64) {
        // Find point (x,y) at corner of cell (i,j).
        x := xyrange * (float64(i)/cells - 0.5)
        y := xyrange * (float64(j)/cells - 0.5)
 
        // Compute surface height z.
        z := f(x, y)
 
        // Project (x,y,z) isometrically onto 2-D SVG canvas (sx,sy).
        sx := width/2 + (x-y)*cos30*xyscale
        sy := height/2 + (x+y)*sin30*xyscale - z*zscale
        return sx, sy
}
 
func f(x, y float64) float64 {
        r := math.Hypot(x, y) // distance from (0,0)
        return math.Sin(r) / r
}