如何通过选择/更改图像kb来调整图像大小？ 而不是分辨率

Question

This is how i'm using it:

Bitmap bmp1 = new Bitmap(ResizeImages(fileName,100,100));

But now i'm changing the images resolution in this example 100,100 But now i want to change the images size by kb i mean if the file size is 3.64MB or for example 3,227KB and i know that in the site i want to upload the images to the maximum size of single image i can upload is 1024k so i want to be able to change the image size to 1024k.

I want to make something like the ResizeImages but instead adding resolution like 100,100 to add a size like ResizeImages(fileName,1024);

This is the ResizeImages method but maybe it's better to make a new method ?

private static Bitmap ResizeImages(String filename, int maxWidth, int maxHeight)
        {
            using (Image originalImage = Image.FromFile(filename))
            {
                //Caluate new Size
                int newWidth = originalImage.Width;
                int newHeight = originalImage.Height;
                double aspectRatio = (double)originalImage.Width / (double)originalImage.Height;
                if (aspectRatio <= 1 && originalImage.Width > maxWidth)
                {
                    newWidth = maxWidth;
                    newHeight = (int)Math.Round(newWidth / aspectRatio);
                }
                else if (aspectRatio > 1 && originalImage.Height > maxHeight)
                {
                    newHeight = maxHeight;
                    newWidth = (int)Math.Round(newHeight * aspectRatio);
                }
                if (newWidth >= 0 && newHeight >= 0)
                {
                    Bitmap newImage = new Bitmap(newWidth, newHeight);
                    using (Graphics g = Graphics.FromImage(newImage))
                    {
                        //--Quality Settings Adjust to fit your application
                        g.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.HighQualityBilinear;
                        g.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.HighQuality;
                        g.PixelOffsetMode = System.Drawing.Drawing2D.PixelOffsetMode.HighQuality;
                        g.CompositingQuality = System.Drawing.Drawing2D.CompositingQuality.HighQuality;
                        g.DrawImage(originalImage, 0, 0, newImage.Width, newImage.Height);
                        return newImage;
                    }
                }
                return null;
            }

UPDATE

I found this two methods and it's working but i didn't understand how it's working with the size change:

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;
        }
        private static void SaveJPGWithCompressionSetting(Image image, string szFileName, long lCompression)
        {
            EncoderParameters eps = new EncoderParameters(1);
            eps.Param[0] = new EncoderParameter(System.Drawing.Imaging.Encoder.Quality, lCompression);
            ImageCodecInfo ici = GetEncoderInfo("image/jpeg");
            image.Save(szFileName, ici, eps);
        }

In the constructor i did:

Bitmap bmp1 = new Bitmap(fileName);
SaveJPGWithCompressionSetting(bmp1, FilePath, 10L);
bmp1.Dispose();

What is the 10L mean ?

In this case for example the fileName original size is 6.71MB The new reszied file is now 566KB

And if i want to make the file size to be 1024k for example ?

Answer 1

So now you use the following code to crate a jpeg file:

Bitmap bmp1 = new Bitmap(fileName);
SaveJPGWithCompressionSetting(bmp1, FilePath, 10L);
bmp1.Dispose();

The 10L will be used for Encoder.Quality , which is a number from 0 to 100, where 100 is the highest quality. The L in 10L just means it is a long , and not an int . So the example above will write a jpeg with quality=10, ie 10%.

As 10% is very low quality, you will probably see that it does not look very good. 75% is a more common value. But with 10% you reduced the filesize from 6.71MB to 566KB, which is a lot. When you use 75%, the file will be much larger.

As you require 1024k, you need to find the correct quality which results in that filesize, or a bit smaller, as you cannot go over the limit. There is no easy way to determine the quality you need. You just have to try it.

So for example, you start with 50%, is the file still too large, you try 25%. You continue this way until you find the best value. It would be the best if you could try this in-memory. But I'm not sure that is possible. Otherwise you just have to write the images to disk, and check the filesize.

It also depends on the image how much the filesize will be reduced by a certain quality. For example a screenshot is much harder to compress (using jpeg) than a nature photograph. So for each picture you will probably have a different quality setting when the filesize if 1024k.