如何在 C# 中将图像大小调整为特定的硬盘大小？

Question

如何将 C# 中的图像大小调整为特定的硬盘大小，例如 2MiB？有没有比反复试验更好的方法（当然，即使它是近似的）。

尝试在网络上查找解决方案时需要搜索哪些特定关键字？

Answer 1

这取决于你愿意改变什么

1. 使图像尺寸变小
2. 更改图像的格式
3. 如果格式支持有损压缩，请降低质量
4. 如果您存储不需要的元数据，请将其删除
5. 减少颜色数量（以及每像素位数）
6. 更改为调色板格式
7. 更改为调色板格式并减少颜色

很难猜测最终的磁盘大小是多少，但如果您知道起点，则可以获得相当好的估计。减小尺寸可能成比例，减少每像素位数也可能成比例。

如果您更改格式、压缩或质量，这实际上只是一种猜测——很大程度上取决于图像内容。您可以通过在与您认为会看到的内容相匹配的图像语料库上进行尝试来获得良好的范围。

Answer 2

可以通过取原始图像尺寸的像素数除以计算图像的近似信息级别：

info = fileSize / (width * height);

我的形象是369636个字节，1200x800像素，因此它使用〜每像素0.385字节。

我有一个较小的版本是101111个字节，600x400像素，因此它使用〜每像素0.4213字节。

当你缩小的图像，你会看到，它通常含有每像素略多的信息，在这种情况下更约9％。根据您的图像的类型和多少你缩小他们，你应该能够计算出多少信息/像素比增大（C）的平均，这样就可以计算出一个近似文件大小：

newFileSize = (fileSize / (width * height)) * (newWidth * newHeight) * c

从这里就可以提取为你如何大，以使图像以达到特定的文件大小的公式：

newWidth * newHeight = (newFileSize / fileSize) * (width * height) / c

这将让你很接近所需的文件大小。如果你想更贴近您可以将图像尺寸调整到计算的大小，压缩它并从你得到的文件大小计算每个像素值的新字节。

Answer 3

我通过降低质量，直到我达到我的期望的尺寸实现这一点。

NB：要求您添加System.Drawing中参考

。

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.IO;
using System.Drawing;
using System.Drawing.Imaging;
using System.Drawing.Drawing2D;

namespace PhotoShrinker
{
class Program
{
/// <summary>
/// Max photo size in bytes
/// </summary>
const long MAX_PHOTO_SIZE = 409600;

static void Main(string[] args)
{
    var photos = Directory.EnumerateFiles(Directory.GetCurrentDirectory(), "*.jpg");

    foreach (var photo in photos)
    {
        var photoName = Path.GetFileNameWithoutExtension(photo);

        var fi = new FileInfo(photo);
        Console.WriteLine("Photo: " + photo);
        Console.WriteLine(fi.Length);

        if (fi.Length > MAX_PHOTO_SIZE)
        {
            using (var image = Image.FromFile(photo)) 
            {
                  using (var stream = DownscaleImage(image))
                  {
                        using (var file = File.Create(photoName + "-smaller.jpg"))
                        {
                            stream.CopyTo(file);
                        }
                  }
            }
            Console.WriteLine("File resized.");
        }
        Console.WriteLine("Done.")
        Console.ReadLine();
    }

}

private static MemoryStream DownscaleImage(Image photo)
{
    MemoryStream resizedPhotoStream = new MemoryStream();

    long resizedSize = 0;
    var quality = 93;
    //long lastSizeDifference = 0;
    do
    {
        resizedPhotoStream.SetLength(0);

        EncoderParameters eps = new EncoderParameters(1);
        eps.Param[0] = new EncoderParameter(System.Drawing.Imaging.Encoder.Quality, (long)quality);
        ImageCodecInfo ici = GetEncoderInfo("image/jpeg");

        photo.Save(resizedPhotoStream, ici, eps);
        resizedSize = resizedPhotoStream.Length;

        //long sizeDifference = resizedSize - MAX_PHOTO_SIZE;
        //Console.WriteLine(resizedSize + "(" + sizeDifference + " " + (lastSizeDifference - sizeDifference) + ")");
        //lastSizeDifference = sizeDifference;
        quality--;

    } while (resizedSize > MAX_PHOTO_SIZE);

    resizedPhotoStream.Seek(0, SeekOrigin.Begin);

    return resizedPhotoStream;
}

private static ImageCodecInfo GetEncoderInfo(String mimeType)
{
    int j;
    ImageCodecInfo[] encoders;
    encoders = ImageCodecInfo.GetImageEncoders();
    for (j = 0; j < encoders.Length; ++j)
    {
        if (encoders[j].MimeType == mimeType)
            return encoders[j];
    }
    return null;
}
}
}

Answer 4

如果它是一个24位BMP我认为你需要做的是这样的：

//initial size =  WxH
long bitsperpixel = 24; //for 24 bit BMP
double ratio;
long size = 2 * 1 << 20;//2MB = 2 * 2^20
size -= 0x35;//subtract the BMP header size from it
long newH, newW, left, right, middle,BMProwsize;
left = 1;
right = size;//binary search for new width and height
while (left < right)
{
    middle = (left + right + 1) / 2;
    newW = middle;
    ratio = Convert.ToDouble(newW) / Convert.ToDouble(W);
    newH = Convert.ToInt64(ratio * Convert.ToDouble(H));
    BMProwsize = 4 * ((newW * bitsperpixel + 31) / 32);
    //row size must be multiple of 4
    if (BMProwsize * newH <= size)
        left = middle;
    else
        right = middle-1;                
}

newW = left;
ratio = Convert.ToDouble(newW) / Convert.ToDouble(W);
newH = Convert.ToInt64(ratio * Convert.ToDouble(H));
//resize image to newW x newH and it should fit in <= 2 MB

如果这是一个不同的BMP类型等的8位的BMP也在标题部分会有更多的数据，其指定从0到255的每个值的实际颜色，所以你需要二进制之前减去从总文件大小更搜索范围。

Answer 5

转换，减少（迭代，在存储器中）＆下载（MVC）

public ActionResult ReduceFileSize(string ImageURL, long MAX_PHOTO_SIZE) //KB
{
    var photo = Server.MapPath("~/" + ImageURL); //Files/somefiles/2018/DOC_82401583cb534b95a10252d29a1eb4ee_1.jpg

    var photoName = Path.GetFileNameWithoutExtension(photo);

    var fi = new FileInfo(photo);

    //const long MAX_PHOTO_SIZE = 100; //KB //109600;

    var MAX_PHOTO_SIZE_BYTES = (MAX_PHOTO_SIZE * 1000);

    if (fi.Length > MAX_PHOTO_SIZE_BYTES)
    {
        using (var image = Image.FromFile(photo))
        {
            using (var mstream = DownscaleImage(image, MAX_PHOTO_SIZE_BYTES))
            {

                //Convert the memorystream to an array of bytes.
                byte[] byteArray = mstream.ToArray();
                //Clean up the memory stream
                mstream.Flush();
                mstream.Close();
                // Clear all content output from the buffer stream
                Response.Clear();
                // Add a HTTP header to the output stream that specifies the default filename
                // for the browser's download dialog
                Response.AddHeader("Content-Disposition", "attachment; filename=" + fi.Name);
                // Add a HTTP header to the output stream that contains the 
                // content length(File Size). This lets the browser know how much data is being transfered
                Response.AddHeader("Content-Length", byteArray.Length.ToString());
                // Set the HTTP MIME type of the output stream
                Response.ContentType = "application/octet-stream";
                // Write the data out to the client.
                Response.BinaryWrite(byteArray);

            }
        }
    }
    else
    {
        return null;
    }

    return null;
}



private static MemoryStream DownscaleImage(Image photo, long MAX_PHOTO_SIZE_BYTES)
{
    MemoryStream resizedPhotoStream = new MemoryStream();

    long resizedSize = 0;
    var quality = 93;
    //long lastSizeDifference = 0;
    do
    {
        resizedPhotoStream.SetLength(0);

        EncoderParameters eps = new EncoderParameters(1);
        eps.Param[0] = new EncoderParameter(System.Drawing.Imaging.Encoder.Quality, (long)quality);
        ImageCodecInfo ici = GetEncoderInfo("image/jpeg");

        photo.Save(resizedPhotoStream, ici, eps);
        resizedSize = resizedPhotoStream.Length;

        //long sizeDifference = resizedSize - MAX_PHOTO_SIZE;
        //Console.WriteLine(resizedSize + "(" + sizeDifference + " " + (lastSizeDifference - sizeDifference) + ")");
        //lastSizeDifference = sizeDifference;
        quality--;

    } while (resizedSize > MAX_PHOTO_SIZE_BYTES);

    resizedPhotoStream.Seek(0, SeekOrigin.Begin);

    return resizedPhotoStream;
}

private static ImageCodecInfo GetEncoderInfo(String mimeType)
{
    int j;
    ImageCodecInfo[] encoders;
    encoders = ImageCodecInfo.GetImageEncoders();
    for (j = 0; j < encoders.Length; ++j)
    {
        if (encoders[j].MimeType == mimeType)
            return encoders[j];
    }
    return null;
}