asp.net - MVC3 WebImage 助手：调整大小正在将透明背景转换为黑色

Question

我正在尝试使用 MVC3 的 WebImage 助手创建缩略图。

原始图像是具有透明背景的 .png。当我尝试使用以下内容调整大小时：

var image = blob.DownloadByteArray();     

new WebImage(image)
    .Resize(50, 50)
    .Write();

生成的缩略图将原来的透明背景替换为黑色背景。

Answer 1

上面的答案很好，但我做了一些微调并实现了图像的“保持比例”，这样我们就不会得到拉伸的图像。

    using System;
using System.Collections.Generic;
using System.Drawing;
using System.Drawing.Imaging;
using System.IO;
using System.Web.Helpers;

public static class ResizePng
{
    private static IDictionary<string, ImageFormat> _transparencyFormats = new Dictionary<string, ImageFormat>(StringComparer.OrdinalIgnoreCase) { { "png", ImageFormat.Png }, { "gif", ImageFormat.Gif } };

    public static WebImage ResizePreserveTransparency(this WebImage image, int width, int height)
    {
        ImageFormat format = null;
        if (!_transparencyFormats.TryGetValue(image.ImageFormat, out format))
        {
            return image.Resize(width, height);
        }

        //keep ratio *************************************
        double ratio = (double)image.Width / image.Height;
        double desiredRatio = (double)width / height;
        if (ratio > desiredRatio)
        {
            height = Convert.ToInt32(width / ratio);
        }
        if (ratio < desiredRatio)
        {
            width = Convert.ToInt32(height * ratio);
        }
        //************************************************

        using (Image resizedImage = new Bitmap(width, height))
        {
            using (Bitmap source = new Bitmap(new MemoryStream(image.GetBytes())))
            {
                using (Graphics g = Graphics.FromImage(resizedImage))
                {
                    g.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.AntiAlias;
                    g.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.HighQualityBicubic;
                    g.DrawImage(source, 0, 0, width, height);
                }
            }
            using (MemoryStream ms = new MemoryStream())
            {
                resizedImage.Save(ms, format);
                return new WebImage(ms.ToArray());
            }
        }
    }

}

Answer 2

您应该为 gif 和 png 图像编写自己的调整大小方法。我为 Web Image 创建了扩展以调整图像大小。请参阅下面我的方法的代码：

public static class WebImageExtension
{
    private static readonly IDictionary<string, ImageFormat> TransparencyFormats =
        new Dictionary<string, ImageFormat>(StringComparer.OrdinalIgnoreCase)
            {{"png", ImageFormat.Png}, {"gif", ImageFormat.Gif}};

    public static WebImage Resize(this WebImage image, int width)
    {
        double aspectRatio = (double)image.Width / image.Height;
        var height = Convert.ToInt32(width/aspectRatio);

        ImageFormat format;

        if (!TransparencyFormats.TryGetValue(image.ImageFormat.ToLower(), out format))
        {
            return image.Resize(width, height);
        }

        using (Image resizedImage = new Bitmap(width, height))
        {
            using (var source = new Bitmap(new MemoryStream(image.GetBytes())))
            {
                using (Graphics g = Graphics.FromImage(resizedImage))
                {
                    g.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.AntiAlias;
                    g.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.HighQualityBicubic;
                    g.DrawImage(source, 0, 0, width, height);
                }
            }

            using (var ms = new MemoryStream())
            {
                resizedImage.Save(ms, format);
                return new WebImage(ms.ToArray());
            }
        }
    }
}

Answer 3

您应该更改.Write以传递您预期的输出类型。它使用此传递的类型来确定要使用的图像类型。

var image = blob.DownloadByteArray();     

new WebImage(image)
    .Resize(50, 50)
    .Write("png");