我想用 Go 嘗試圖形庫。我找到了以下示例：package mainimport (    "image"    "image/color"    "image/png"    "log"    "os")func main() {    width, height := 512, 512    canvas := NewCanvas(image.Rect(0, 0, width, height))    canvas.DrawGradient()    // Draw a series of lines from the top left corner to the bottom of the image    for x := 0; x < width; x += 8 {        canvas.DrawLine(color.RGBA{0, 0, 0, 255},            Vector{0.0, 0.0},            Vector{float64(x), float64(height)})    }    outFilename := "lines.png"    outFile, err := os.Create(outFilename)    if err != nil {        log.Fatal(err)    }    defer outFile.Close()    log.Print("Saving image to: ", outFilename)    png.Encode(outFile, canvas)}但是，在構建時似乎缺少某些類。D:\go\work>go build draw.go# command-line-arguments.\draw.go:13: undefined: NewCanvas.\draw.go:19: undefined: Vector.\draw.go:20: undefined: Vector我的環(huán)境在 HelloWorld 示例中運行良好，但是在導入圖像庫時似乎缺少某些內容。對新手入門有什么幫助嗎？