创建缩略图

Question

我想在文件位置的网格视图中显示缩略图。如何生成.jpeg 文件？ 我正在使用C# 语言和asp.net。

Answer 1

你必须在Image类中使用GetThumbnailImage方法：

https://msdn.microsoft.com/en-us/library/8t23aykb%28v=vs.110%29.aspx

这是一个粗略的示例，它获取图像文件并从中制作缩略图，然后将其保存回磁盘。

Image image = Image.FromFile(fileName);
Image thumb = image.GetThumbnailImage(120, 120, ()=>false, IntPtr.Zero);
thumb.Save(Path.ChangeExtension(fileName, "thumb"));

它在 System.Drawing 命名空间中（在 System.Drawing.dll 中）。

行为：

如果 Image 包含嵌入的缩略图，此方法 检索嵌入的缩略图并将其缩放到请求的大小。 如果 Image 不包含嵌入的缩略图，则此方法 通过缩放主图像创建缩略图。

---

重要提示：上述 Microsoft 链接的备注部分警告某些潜在问题：

GetThumbnailImage 方法在请求缩略图时效果很好 图像的大小约为 120 x 120 像素。如果您要求大 缩略图（例如，300 x 300）来自具有 嵌入的缩略图，可能会出现明显的质量损失 缩略图。

缩放主图像可能会更好（而不是 通过调用DrawImage 方法来缩放嵌入的缩略图）。

Answer 2

以下代码将根据响应写入一个图像，您可以根据自己的目的修改代码：

public void WriteImage(string path, int width, int height)
{
    Bitmap srcBmp = new Bitmap(path);
    float ratio = srcBmp.Width / srcBmp.Height;
    SizeF newSize = new SizeF(width, height * ratio);
    Bitmap target = new Bitmap((int) newSize.Width,(int) newSize.Height);
    HttpContext.Response.Clear();
    HttpContext.Response.ContentType = "image/jpeg";
    using (Graphics graphics = Graphics.FromImage(target))
    {
        graphics.CompositingQuality = CompositingQuality.HighSpeed;
        graphics.InterpolationMode = InterpolationMode.HighQualityBicubic;
        graphics.CompositingMode = CompositingMode.SourceCopy;
        graphics.DrawImage(srcBmp, 0, 0, newSize.Width, newSize.Height);
        using (MemoryStream memoryStream = new MemoryStream()) 
        {
            target.Save(memoryStream, ImageFormat.Jpeg);
            memoryStream.WriteTo(HttpContext.Response.OutputStream);
        }
    }
    Response.End();
}

Answer 3

这是一个如何创建较小图像（缩略图）的完整示例。这个 sn-p 调整图像的大小，在需要时旋转它（如果垂直握住手机）并填充图像，如果你想创建方形拇指。这个 sn-p 创建一个 JPEG，但它可以很容易地修改为其他文件类型。即使图像小于允许的最大尺寸，图像仍会被压缩，并且会更改其分辨率以创建具有相同 dpi 和压缩级别的图像。

using System;
using System.Drawing;
using System.Drawing.Drawing2D;
using System.Drawing.Imaging;
using System.IO;

//set the resolution, 72 is usually good enough for displaying images on monitors
float imageResolution = 72;

//set the compression level. higher compression = better quality = bigger images
long compressionLevel = 80L;


public Image resizeImage(Image image, int maxWidth, int maxHeight, bool padImage)
{
    int newWidth;
    int newHeight;

    //first we check if the image needs rotating (eg phone held vertical when taking a picture for example)
    foreach (var prop in image.PropertyItems)
    {
        if (prop.Id == 0x0112)
        {
            int orientationValue = image.GetPropertyItem(prop.Id).Value[0];
            RotateFlipType rotateFlipType = getRotateFlipType(orientationValue);
            image.RotateFlip(rotateFlipType);
            break;
        }
    }

    //apply the padding to make a square image
    if (padImage == true)
    {
        image = applyPaddingToImage(image, Color.Red);
    }

    //check if the with or height of the image exceeds the maximum specified, if so calculate the new dimensions
    if (image.Width > maxWidth || image.Height > maxHeight)
    {
        double ratioX = (double)maxWidth / image.Width;
        double ratioY = (double)maxHeight / image.Height;
        double ratio = Math.Min(ratioX, ratioY);

        newWidth = (int)(image.Width * ratio);
        newHeight = (int)(image.Height * ratio);
    }
    else
    {
        newWidth = image.Width;
        newHeight = image.Height;
    }

    //start the resize with a new image
    Bitmap newImage = new Bitmap(newWidth, newHeight);

    //set the new resolution
    newImage.SetResolution(imageResolution, imageResolution);

    //start the resizing
    using (var graphics = Graphics.FromImage(newImage))
    {
        //set some encoding specs
        graphics.CompositingMode = CompositingMode.SourceCopy;
        graphics.CompositingQuality = CompositingQuality.HighQuality;
        graphics.SmoothingMode = SmoothingMode.HighQuality;
        graphics.InterpolationMode = InterpolationMode.HighQualityBicubic;
        graphics.PixelOffsetMode = PixelOffsetMode.HighQuality;

        graphics.DrawImage(image, 0, 0, newWidth, newHeight);
    }

    //save the image to a memorystream to apply the compression level
    using (MemoryStream ms = new MemoryStream())
    {
        EncoderParameters encoderParameters = new EncoderParameters(1);
        encoderParameters.Param[0] = new EncoderParameter(Encoder.Quality, compressionLevel);

        newImage.Save(ms, getEncoderInfo("image/jpeg"), encoderParameters);

        //save the image as byte array here if you want the return type to be a Byte Array instead of Image
        //byte[] imageAsByteArray = ms.ToArray();
    }

    //return the image
    return newImage;
}


//=== image padding
public Image applyPaddingToImage(Image image, Color backColor)
{
    //get the maximum size of the image dimensions
    int maxSize = Math.Max(image.Height, image.Width);
    Size squareSize = new Size(maxSize, maxSize);

    //create a new square image
    Bitmap squareImage = new Bitmap(squareSize.Width, squareSize.Height);

    using (Graphics graphics = Graphics.FromImage(squareImage))
    {
        //fill the new square with a color
        graphics.FillRectangle(new SolidBrush(backColor), 0, 0, squareSize.Width, squareSize.Height);

        //put the original image on top of the new square
        graphics.DrawImage(image, (squareSize.Width / 2) - (image.Width / 2), (squareSize.Height / 2) - (image.Height / 2), image.Width, image.Height);
    }

    //return the image
    return squareImage;
}


//=== get encoder info
private ImageCodecInfo getEncoderInfo(string mimeType)
{
    ImageCodecInfo[] encoders = ImageCodecInfo.GetImageEncoders();

    for (int j = 0; j < encoders.Length; ++j)
    {
        if (encoders[j].MimeType.ToLower() == mimeType.ToLower())
        {
            return encoders[j];
        }
    }

    return null;
}


//=== determine image rotation
private RotateFlipType getRotateFlipType(int rotateValue)
{
    RotateFlipType flipType = RotateFlipType.RotateNoneFlipNone;

    switch (rotateValue)
    {
        case 1:
            flipType = RotateFlipType.RotateNoneFlipNone;
            break;
        case 2:
            flipType = RotateFlipType.RotateNoneFlipX;
            break;
        case 3:
            flipType = RotateFlipType.Rotate180FlipNone;
            break;
        case 4:
            flipType = RotateFlipType.Rotate180FlipX;
            break;
        case 5:
            flipType = RotateFlipType.Rotate90FlipX;
            break;
        case 6:
            flipType = RotateFlipType.Rotate90FlipNone;
            break;
        case 7:
            flipType = RotateFlipType.Rotate270FlipX;
            break;
        case 8:
            flipType = RotateFlipType.Rotate270FlipNone;
            break;
        default:
            flipType = RotateFlipType.RotateNoneFlipNone;
            break;
    }

    return flipType;
}


//== convert image to base64
public string convertImageToBase64(Image image)
{
    using (MemoryStream ms = new MemoryStream())
    {
        //convert the image to byte array
        image.Save(ms, ImageFormat.Jpeg);
        byte[] bin = ms.ToArray();

        //convert byte array to base64 string
        return Convert.ToBase64String(bin);
    }
}

为 asp.net 用户提供一个关于如何上传文件、调整文件大小并在页面上显示结果的小示例。

//== the button click method
protected void Button1_Click(object sender, EventArgs e)
{
    //check if there is an actual file being uploaded
    if (FileUpload1.HasFile == false)
    {
        return;
    }

    using (Bitmap bitmap = new Bitmap(FileUpload1.PostedFile.InputStream))
    {
        try
        {
            //start the resize
            Image image = resizeImage(bitmap, 256, 256, true);

            //to visualize the result, display as base64 image
            Label1.Text = "<img src=\"data:image/jpg;base64," + convertImageToBase64(image) + "\">";

            //save your image to file sytem, database etc here
        }
        catch (Exception ex)
        {
            Label1.Text = "Oops! There was an error when resizing the Image.<br>Error: " + ex.Message;
        }
    }
}

Answer 4

这是一个将高分辨率图像转换为缩略图大小的示例-

protected void Button1_Click(object sender, EventArgs e)
{
    //----------        Getting the Image File
    System.Drawing.Image img = System.Drawing.Image.FromFile(Server.MapPath("~/profile/Avatar.jpg"));

    //----------        Getting Size of Original Image
    double imgHeight = img.Size.Height;
    double imgWidth = img.Size.Width;

    //----------        Getting Decreased Size
    double x = imgWidth / 200;
    int newWidth = Convert.ToInt32(imgWidth / x);
    int newHeight = Convert.ToInt32(imgHeight / x);

    //----------        Creating Small Image
    System.Drawing.Image.GetThumbnailImageAbort myCallback = new System.Drawing.Image.GetThumbnailImageAbort(ThumbnailCallback);
    System.Drawing.Image myThumbnail = img.GetThumbnailImage(newWidth, newHeight, myCallback, IntPtr.Zero);

    //----------        Saving Image
    myThumbnail.Save(Server.MapPath("~/profile/NewImage.jpg"));
}
public bool ThumbnailCallback()
{
    return false;
}

来源- http://iknowledgeboy.blogspot.in/2014/03/c-creating-thumbnail-of-large-image-by.html

Answer 5

这是基于已接受答案的版本。它解决了两个问题...

1. 图像处理不当。
2. 保持图像的纵横比。

我发现这个工具对 JPG 和 PNG 文件都快速有效。

private static FileInfo CreateThumbnailImage(string imageFileName, string thumbnailFileName)
{
    const int thumbnailSize = 150;
    using (var image = Image.FromFile(imageFileName))
    {
        var imageHeight = image.Height;
        var imageWidth = image.Width;
        if (imageHeight > imageWidth)
        {
            imageWidth = (int) (((float) imageWidth / (float) imageHeight) * thumbnailSize);
            imageHeight = thumbnailSize;
        }
        else
        {
            imageHeight = (int) (((float) imageHeight / (float) imageWidth) * thumbnailSize);
            imageWidth = thumbnailSize;
        }

        using (var thumb = image.GetThumbnailImage(imageWidth, imageHeight, () => false, IntPtr.Zero))
            //Save off the new thumbnail
            thumb.Save(thumbnailFileName);
    }

    return new FileInfo(thumbnailFileName);
}

Answer 6

这是我正在使用的代码。也适用于 .NET Core > 2.0 使用 System.Drawing.Common NuGet。

https://www.nuget.org/packages/System.Drawing.Common/

using System;
using System.Drawing;

class Program
{
    static void Main()
    {
        const string input = "C:\\background1.png";
        const string output = "C:\\thumbnail.png";

        // Load image.
        Image image = Image.FromFile(input);

        // Compute thumbnail size.
        Size thumbnailSize = GetThumbnailSize(image);

        // Get thumbnail.
        Image thumbnail = image.GetThumbnailImage(thumbnailSize.Width,
            thumbnailSize.Height, null, IntPtr.Zero);

        // Save thumbnail.
        thumbnail.Save(output);
    }

    static Size GetThumbnailSize(Image original)
    {
        // Maximum size of any dimension.
        const int maxPixels = 40;

        // Width and height.
        int originalWidth = original.Width;
        int originalHeight = original.Height;

        // Return original size if image is smaller than maxPixels
        if (originalWidth <= maxPixels || originalHeight <= maxPixels)
        {
            return new Size(originalWidth, originalHeight);
        }   

        // Compute best factor to scale entire image based on larger dimension.
        double factor;
        if (originalWidth > originalHeight)
        {
            factor = (double)maxPixels / originalWidth;
        }
        else
        {
            factor = (double)maxPixels / originalHeight;
        }

        // Return thumbnail size.
        return new Size((int)(originalWidth * factor), (int)(originalHeight * factor));
    }
}

来源：

https://www.dotnetperls.com/getthumbnailimage