使用 Graphics.DrawImage ，将我的透明 PNG 图像的背景更改为黑色

Question

我已经编写了这个方法来调整我的图像大小，但是我的透明 PNG 图像的返回图像具有黑色背景。 解决办法是什么？

我尝试过Bitmap.MakeTransparent() 和Graphics.Clear()，但无法解决我的问题。 我检查了所有相关问题，但找不到任何有用的答案。

public HttpResponseMessage ImageResizer(string path, int w,int h)
    {

        var imagePath = HttpContext.Current.Server.MapPath(path); 
        Bitmap image;
        try
        {
            image = (Bitmap)System.Drawing.Image.FromFile(imagePath);
        }
        catch
        {
            HttpResponseMessage hrm = new HttpResponseMessage();
            return hrm;
        }

        int originalWidth = image.Width;
        int originalHeight = image.Height;

        // New width and height based on aspect ratio
        int newWidth = w;
        int newHeight = h;

        // Convert other formats (including CMYK) to RGB.
        Bitmap newImage = new Bitmap(newWidth, newHeight, PixelFormat.Format24bppRgb);
        // Draws the image in the specified size with quality mode set to HighQuality
        using (Graphics graphics = Graphics.FromImage(newImage))
        {
            graphics.InterpolationMode = InterpolationMode.HighQualityBicubic;
            graphics.SmoothingMode = SmoothingMode.AntiAlias;
            graphics.CompositingQuality = CompositingQuality.HighQuality;
            graphics.PixelOffsetMode = PixelOffsetMode.HighQuality;

            using (var attribute = new ImageAttributes())
            {
                attribute.SetWrapMode(WrapMode.TileFlipXY);

                // draws the resized image to the bitmap
                graphics.DrawImage(image, new Rectangle(0, 0, newImage.Width, newImage.Height), new Rectangle(0, 0, originalWidth, originalHeight), GraphicsUnit.Pixel);
            }
        }
        // Get an ImageCodecInfo object that represents the PNG codec.
        ImageCodecInfo imageCodecInfo = this.GetEncoderInfo(ImageFormat.Png);

        // Create an Encoder object for the Quality parameter.
        Encoder encoder = Encoder.Quality;

        // Create an EncoderParameters object. 
        EncoderParameters encoderParameters = new EncoderParameters(1);

        // Save the image as a PNG file with quality level.
        EncoderParameter encoderParameter = new EncoderParameter(encoder, 10);
        encoderParameters.Param[0] = encoderParameter;
        var splitPath = imagePath.Split('.');
        string newPath = splitPath[0] + "ss5.png";
        newImage.Save(newPath, ImageFormat.Png);

        MemoryStream memoryStream = new MemoryStream();
        newImage.Save(memoryStream, System.Drawing.Imaging.ImageFormat.Png);
        HttpResponseMessage response = new HttpResponseMessage();
        response.Content = new ByteArrayContent(memoryStream.ToArray());
        response.Content.Headers.ContentType = new System.Net.Http.Headers.MediaTypeHeaderValue("image/png");
        response.Content.Headers.ContentLength = memoryStream.Length;
        return response;
    }

    private ImageCodecInfo GetEncoderInfo(ImageFormat format)
    {
        return ImageCodecInfo.GetImageDecoders().SingleOrDefault(c => c.FormatID == format.Guid);
    }

Answer 1

您在 Bitmap 构造函数中使用了 PixelFormat.Format24bppRgb。该格式将您的位图限制为 3 个通道：红色、绿色和蓝色。因此，您正在创建的位图不支持 alpha（也称为透明度），并将默认为纯黑色图像。当您绘制具有透明度的图像时，该图像的 alpha 将被预乘或丢弃，具体取决于其格式。

如果你想用透明保存你的新图像，你需要用PixelFormat.Format32bppArgb声明它：

Bitmap newImage = new Bitmap(newWidth, newHeight, PixelFormat.Format32bppArgb);