PNG格式图片常见转换方法

前言

最近碰到一个需要将图片由原始的PNG转化为JPG的需求，由于PNG图片本身质量等原因，导致转化为JPG之后，存在失真的问题，后来一个同事分享了下述的HighQualityPNGToJPG方法解决PNG转JPG失真的问题。

1、准备

using System.Drawing;
using System.Net;
using System.Drawing.Imaging;

/// <summary>
/// 常见图片格式枚举
/// </summary>
public enum ImageFormatenmu
{
    jpg,
    gif,
    bmp,
    png
}

2、给透明PNG图片添加白色底图

/// <summary>
/// 透明PNG图片添加白色底图
/// </summary>
/// <param name="pngFilePath">Png文件路径</param>
/// <param name="savePngFilePath">新Png文件保存路径</param>
/// <returns></returns>
public static void AddBlankBaseMap(string pngFilePath, string savePngFilePath)
{
    Bitmap bitmap = new Bitmap(pngFilePath);
    int w = bitmap.Width;
    int h = bitmap.Height;
    Bitmap dstBitmap = new Bitmap(bitmap.Width, bitmap.Height, System.Drawing.Imaging.PixelFormat.Format32bppArgb);
    System.Drawing.Imaging.BitmapData srcData = bitmap.LockBits(new Rectangle(0, 0, w, h), System.Drawing.Imaging.ImageLockMode.ReadOnly, System.Drawing.Imaging.PixelFormat.Format32bppArgb);
    System.Drawing.Imaging.BitmapData dstData = dstBitmap.LockBits(new Rectangle(0, 0, w, h), System.Drawing.Imaging.ImageLockMode.WriteOnly, System.Drawing.Imaging.PixelFormat.Format32bppArgb);
    //不安全代码，编译时需将项目生成操作中添加【允许不安全代码】
    unsafe
    {
        byte* pIn = (byte*)srcData.Scan0.ToPointer();
        byte* pOut = (byte*)dstData.Scan0.ToPointer();
        byte* p;
        int stride = srcData.Stride;
        int r, g, b, a;
        for (int y = 0; y < h; y++)
        {
            for (int x = 0; x < w; x++)
            {
                p = pIn;
                b = pIn[0];
                g = pIn[1];
                r = pIn[2];
                a = pIn[3];
                if (a == 0)
                {
                    pOut[1] = (byte)255;
                    pOut[2] = (byte)255;
                    pOut[3] = (byte)255;
                    pOut[0] = (byte)255;
                }
                else
                {
                    pOut[1] = (byte)g;
                    pOut[2] = (byte)r;
                    pOut[3] = (byte)a;
                    pOut[0] = (byte)b;
                }
                pIn += 4;
                pOut += 4;
            }
            pIn += srcData.Stride - w * 4;
            pOut += srcData.Stride - w * 4;
        }
        bitmap.UnlockBits(srcData);
        dstBitmap.UnlockBits(dstData);
        //也可以再这里讲dstBitmap转化为其他类型，如转gif：bitmap.Save(saveFilePath, ImageFormat.Gif)
        dstBitmap.Save(savePngFilePath);
    }
}

3、根据图片url保存图片

/// <summary>
/// 根据图片url保存图片
/// </summary>
/// <param name="url">url地址</param>
/// <param name="savePath">保存本地路径【含文件名，注意扩展名要和url对应文件本身一样，否则可能出错】</param>
public static void GetImageByURL(String url, String savePath)
{
    WebClient webClient = new WebClient();
    webClient.DownloadFile(url, savePath);
}

4、普通PNG格式转换

/// <summary>
/// 普通PNG格式转换
/// </summary>
/// <param name="pngFilePath">png文件路径</param>
/// <param name="saveFilePath">保存文件路径</param>
/// <param name="imageFromat">转化格式</param>
public static void GeneralPngChange(String pngFilePath,String saveFilePath,ImageFormatenmu imageFromat)
{
    Bitmap bitmap = new Bitmap(pngFilePath);
    switch (imageFromat)
    {
        case ImageFormatenmu.bmp:
            {
                bitmap.Save(saveFilePath, ImageFormat.Bmp);
            };
            break;
        case ImageFormatenmu.gif:
            {
                bitmap.Save(saveFilePath, ImageFormat.Gif);
            };
            break;
        case ImageFormatenmu.jpg:
            {
                bitmap.Save(saveFilePath, ImageFormat.Jpeg);
            };
            break;
    }
}

5、PNG转JPG一般操作方法,可设置图片质量

/// <summary>
/// PNG转JPG一般操作方法,可设置图片质量
/// </summary>
/// <param name="pngFilePath">png文件路径</param>
/// <param name="saveFilePath">jpg文件路径</param>
/// <param name="percentage">图片质量>0</param>
public static void GeneralPngToJpg(String pngFilePath, String saveFilePath, long percentage)
{
    Bitmap bmp1 = new Bitmap(pngFilePath);
    ImageCodecInfo jgpEncoder = GetEncoder(ImageFormat.Jpeg);
    System.Drawing.Imaging.Encoder myEncoder =System.Drawing.Imaging.Encoder.Quality;
    EncoderParameters myEncoderParameters = new EncoderParameters(1);
    EncoderParameter myEncoderParameter = new EncoderParameter(myEncoder, percentage);
    myEncoderParameters.Param[0] = myEncoderParameter;
    bmp1.Save(saveFilePath, jgpEncoder, myEncoderParameters);
}

/// <summary>
/// 获取图片编码信息
/// </summary>
/// <param name="mimeType">图片格式MIME类型</param>
/// <returns></returns>
private static ImageCodecInfo GetEncoder(ImageFormat format)
{
    ImageCodecInfo[] codecs = ImageCodecInfo.GetImageDecoders();
    foreach (ImageCodecInfo codec in codecs)
    {
        if (codec.FormatID == format.Guid)
        {
            return codec;
        }
    }
    return null;
}

6、高保真PNG转JPG，解决部分PNG转JPG过程中图像失真的问题

/// <summary>
/// 高保真PNG转JPG，解决部分PNG转JPG过程中图像失真的问题
/// 例如：本人将一张透明的PNG转JPG后，JEP图片失真
/// </summary>
/// <param name="pngFilePath">png文件路径</param>
/// <param name="jpgFilePath">jpg文件路径</param>
public static void HighQualityPNGToJPG(String pngFilePath, String jpgFilePath)
{
    Image pngImg = Image.FromFile(pngFilePath);
    var width = pngImg.Width;
    var height = pngImg.Height;
    Image bitmap = new System.Drawing.Bitmap(width, height);
    Graphics g = System.Drawing.Graphics.FromImage(bitmap);
    //设置高质量插值法
    g.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.HighQualityBicubic;
    g.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.HighQuality;
    g.Clear(Color.White);
    //在指定位置并且按指定大小绘制原图片的指定部分
    g.DrawImage(pngImg, new Rectangle(0, 0, width, height), new Rectangle(0, 0, width, height), GraphicsUnit.Pixel);
    bitmap.Save(jpgFilePath, System.Drawing.Imaging.ImageFormat.Jpeg);
    bitmap.Dispose();
    g.Dispose();
}
}

我遇到将一个透明的PNG或一个质量较低的PNG用上述方法转JPG出现失真的问题。

例如：

使用GeneralPngToJpg方法将png图转jpg，透明底图PNG转换情况如下：

图表 1 透明底图PNG

图表 2透明底图PNG转化后的JPG图片【较严重失真】

非透明底图PNG转换情况如下：

图表 3非透明底图PNG

图表 4 非透明底图PNG转JPG【较严重失真】

HighQualityPNGToJPG分别转化透明及非透明低图的PNG，结果如下：

图表 5 非透明底图的PNG格式转化后的JPG

图表 6 透明底图的PNG格式转化后的JPG

7、测试代码

String imageDirectory = string.Format("{0}image/",AppDomain.CurrentDomain.BaseDirectory);

//透明PNG图片添加白色底图
String transparentPng = Path.Combine(imageDirectory, "transparent.png");
String noTransparentPng = Path.Combine(imageDirectory, "noTransparent.png");
PngHelp.AddBlankBaseMap(transparentPng, noTransparentPng);

//根据图片url保存图片
String urlPng=Path.Combine(imageDirectory, "url.png");
PngHelp.GetImageByURL("http://image.bitauto.com/dealer/brandgroup/m/389.png", urlPng);

//PNG转JPG一般操作方法【存在失真的情况，原因决定于原始PNG图片的质量】
String changeJpg=Path.Combine(imageDirectory, "changeJpg.jpg");
PngHelp.GeneralPngChange(noTransparentPng, changeJpg,ImageFormatenmu.jpg);
String changeBmp = Path.Combine(imageDirectory, "changeBmp.bmp");
PngHelp.GeneralPngChange(noTransparentPng, changeBmp, ImageFormatenmu.bmp);
String changeGif = Path.Combine(imageDirectory, "changeGif.gif");
PngHelp.GeneralPngChange(noTransparentPng, changeBmp, ImageFormatenmu.gif);
            
//【透明图失真更多】
String changeNewJpg = Path.Combine(imageDirectory, "changeNewJpg.jpg");
PngHelp.GeneralPngChange(transparentPng, changeNewJpg, ImageFormatenmu.jpg);

//PNG转JPG一般操作方法,可设置图片质量
String changeJpg0=Path.Combine(imageDirectory, "changeJpg0.jpg");
PngHelp.GeneralPngToJpg(noTransparentPng, changeJpg0,0L);
String changeJpg50 = Path.Combine(imageDirectory, "changeJpg50.jpg");
PngHelp.GeneralPngToJpg(noTransparentPng, changeJpg50, 50L);
String changeJpg100 = Path.Combine(imageDirectory, "changeJpg100.jpg");
PngHelp.GeneralPngToJpg(noTransparentPng, changeJpg100, 50L);

//高保真PNG转JPG，解决部分PNG转JPG过程中图像失真的问题
String highqualityJpgFromTrasparent = Path.Combine(imageDirectory, "highqualityJpgFromTrasparent.jpg");
String highqualityJpgFromNoTrasparent = Path.Combine(imageDirectory, "highqualityJpgFromNoTrasparent.jpg");
PngHelp.HighQualityPNGToJPG(transparentPng, highqualityJpgFromTrasparent);
PngHelp.HighQualityPNGToJPG(noTransparentPng, highqualityJpgFromNoTrasparent);

8、完整示例代码

点击这里下载

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作者：ChowYy

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