检查png图像在戈朗中是否全部透明

Question

我有多个像这样的网址，总是返回PNG图像。https://hydro.nationalmap.gov/arcgis/rest/services/wbd/MapServer/export?bbox=-106.6462299999999885,25.83722399999999908,-93.50780600000010168,36.50038700000000347&size=640,519&bboxSR=4326&layers=show:4&f=image&transparent=true我想检查从URL返回的图像是否像上面的URL一样都是空的（透明的），或者它里面有一些实际的图像。我通过以下函数发出请求，并检查HTTP状态是否为200，返回的内容类型是否为图像。我需要在此处添加功能以测试图像是否为空。thumbnail := "https://hydro.nationalmap.gov/arcgis/rest/services/wbd/MapServer/export?bbox=-106.6462299999999885,25.83722399999999908,-93.50780600000010168,36.50038700000000347&size=640,519&bboxSR=4326&layers=show:4&f=image&transparent=true"resp, err := client.Get(thumbnail)if err != nil {    fmt.Println(err)} else if resp.StatusCode == 200 && strings.HasPrefix(resp.Header["Content-Type"][0], "image") {    return thumbnail} else {    fmt.Println(thumbnail, resp.StatusCode, resp.Header["Content-Type"][0])}

Answer 1

标准库提供了功能强大的软件包和解码器，使我们能够非常轻松地做到这一点。imageimage/png

我们知道透明度意味着 alpha=0，我们需要做的就是迭代图像的像素。

package main

import (

    "image/png"

    "io"

    "log"

    "net/http"

)

func main() {

    transparentResp, _ := http.Get("https://upload.wikimedia.org/wikipedia/commons/3/3d/1_120_transparent.png")

    defer transparentResp.Body.Close()

    notTransparentResp, _ := http.Get("https://upload.wikimedia.org/wikipedia/commons/thumb/5/58/Pitch.png/640px-Pitch.png")

    defer notTransparentResp.Body.Close()

    println(isFullyTransparentPng(transparentResp.Body))

    println(isFullyTransparentPng(notTransparentResp.Body))

}

func isFullyTransparentPng(reader io.Reader) bool {

    img, _ := png.Decode(reader)

    for x := img.Bounds().Min.X; x < img.Bounds().Dx(); x++ {

        for y := img.Bounds().Min.Y; y < img.Bounds().Dy(); y++ {

            _, _, _, alpha := img.At(x, y).RGBA()

            if alpha != 0 {

                return false

            }

        }

    }

    return true

}