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Gunnar Peipman

Resizing images without loss of quality

ASP.NET provides us with System.Drawing namespace where we can find classes we can use to manipulate with images. There are many people out there who mistakenly think thatImage.GetThumbnailImage is best choice for image resizing. You can easily create crappy images if you follow the code examples in previously pointed article. In this post I will show you how to resize images without negative side effects.

Some words about images and thumbnails

Cameras today are powerful tools. They can do many tricks we cannot even imagine if we are dumb users. One those functionalities is generating thumbnails of images. These thumbnails are embedded in original image and are usually small by their dimensions.

If you take image with high resolution and resize it to 50% then using Image.GetThumbnailImage method may produce the result we expect. It takes small thumbnail image and resizes it larger. This causes hard loss of quality and resized image look awful.

Our original image here is taken during Wacken Open Air 2005. I am on this photo too (guess which one). Original image has dimensions 2048×1536. Be aware – size this file is about 1.6MB.


Click on the image to see it in original size.

This file contains thumbnail image and let’s see how previously pointed example fails.

Image.GetThumbnailImage

As a first thing let’s use Image.GetThumbnailImage to get new version of original image that is four times smaller (512×384). Here is the code.

public void ResizeImage(double scaleFactor, Stream fromStream, Stream toStream)
{
    var image = Image.FromStream(fromStream);
    var newWidth = (int)(image.Width * scaleFactor);
    var newHeight = (int)(image.Height * scaleFactor);
 
    var abort = new Image.GetThumbnailImageAbort(ThumbnailCallback);
    var thumbnail = image.GetThumbnailImage(newWidth, newHeight, abort, IntPtr.Zero);
    thumbnail.Save(toStream, image.RawFormat);
 
    thumbnail.Dispose();
    image.Dispose();
}
 
public bool ThumbnailCallback()
{
    return false;
}

And here is the result…

Awful, isn’t it? Now I tell you a little secret. It is directly copy-pasted from MSDN library:

If the Image contains an embedded thumbnail image, this method retrieves the embedded thumbnail and scales it to the requested size. If the Image does not contain an embedded thumbnail image, this method creates a thumbnail image by scaling the main image.

The GetThumbnailImage method works well when the requested thumbnail image has a size of about 120 x 120 pixels. If you request a large thumbnail image (for example, 300 x 300) from an Image that has an embedded thumbnail, there could be a noticeable loss of quality in the thumbnail image. It might be better to scale the main image (instead of scaling the embedded thumbnail) by calling the DrawImage method.

I have on advice too: don’t trust everything you find in web, even if it has very high position in search engine results (Google: asp.net resize image).

Let’s follow now MSDN Library suggestion and make thumbnail that is 7% of original image. To get image with this size you should use 0.07 as scaling factor.

Use GetThumbnailImage to create small thumbnails. It has less impact on your system and it wants less resources than resizing full size image down to thumbnail size.

Custom resizing

Now let’s see the code that makes clean resizing. This code doesn’t use GetThumbnailImage method and operates therefore on full size image. Also you can see that this code tries to save as much quality as possible.

public void ResizeImage(double scaleFactor, Stream fromStream, Stream toStream)
{
    var image = Image.FromStream(fromStream);
    var newWidth = (int)(image.Width * scaleFactor);
    var newHeight = (int)(image.Height * scaleFactor);
    var thumbnailBitmap = new Bitmap(newWidth, newHeight);
 
    var thumbnailGraph = Graphics.FromImage(thumbnailBitmap);
    thumbnailGraph.CompositingQuality = CompositingQuality.HighQuality;
    thumbnailGraph.SmoothingMode = SmoothingMode.HighQuality;
    thumbnailGraph.InterpolationMode = InterpolationMode.HighQualityBicubic;
 
    var imageRectangle = new Rectangle(0, 0, newWidth, newHeight);
    thumbnailGraph.DrawImage(image, imageRectangle);
 
    thumbnailBitmap.Save(toStream, image.RawFormat);
 
    thumbnailGraph.Dispose();
    thumbnailBitmap.Dispose();
    image.Dispose();
}

And the resulting picture is here.

This image is larger than thumbnail, but still looks nice.

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Categories: C# Graphics

View Comments (53)

  • Thank you for your comment, Justin. :)

    I will investigate BitmapSource class as soon as I find time for it. If it makes life easier then I will blog it about for sure.

  • Sorry for newbie question. How would you call this class to resize to thumbnail? Thank you.

  • Can you show the toStream - I get a null exception and wonder how to pass this infomation back out and post it to the page for display.

  • everything seems nice, but draw image sometimes doesn't resize as it should and instead of having a width of 300 for instance, you might end up with a width of 295... drawimage has some major bugs!

  • Thanks for feedback, Nathanael!

    I suggest also other readers take a look at links Nathanael provided here. First one is really *GOOD* reading. :)

  • i have a photo that is uploading like ~/images/1.jpg. how can i send this photo to this void class? in the other hand , how can i use this class? how prepare inputs (Stream)?

  • what are the namespace should be included for this method? i just convert it to the VB code. but it isshowing error on "Dim thumbnailBitmap = New Bitmap(newWidth, newHeight)". what to do ?

  • Thanks man

    Great code

    have been searching for this since very long time

    but your blog has unique solution which was not present anywhere in the web.

    Thank you very much for such a great post

  • This is really very nice. I really appreciate this post.

    How can I dynamically calculate the scaleFactor from the size of the original image?

    This is necessary because I want my thumbnails to have a max height/width, which this example does not give.

  • You can calculate it in caller method by example. The other option is to bring this calculation to resizing method. But I don't think it is an good idea because calculating scale factor is not responsibility of resize image method.

  • Thanks, I will give it a shot, I don't want to calculate it within the resize method, my calling method will calculate and pass it to the resize method.

    My challenge however lies with the fomula to calculate the scale factor, given original image height, width and maximum width/height.

    eg. I want to create my thumbnails to have a maximum width/height of 100 irrespective of the different sizes of images my users may upload.

    I know this is not within the scope of this post, I need help on this and will really appreciate it.

  • I've gotten my answer. To calculate the scale factor of an image, you divide the max width/height by the larger of the height and width. Example (VB.NET):

    Dim max As Integer = 100

    If (img.Width >= img.Height) Then
    scaleFactor = (100 / img.Width)
    Else
    scaleFactor = (100 / img.Height)
    End If

    newWidth = CInt(img.Width * scaleFactor)
    newHeight = CInt(img.Height * scaleFactor)

    I hope this is helpful to someone else out there

  • wow, that example got a standing ovation!!!
    It gave me a square thumbnail, perfect for my project.

    Thanxxxx.

  • Hey,

    I think your work is awesome, going by what i see as output and readers comments.

    I tried some other examples, and i am completely new to images part, i am just a DB guy.

    So can you help me in baby steps if you dont mind

    I have an image ~/images/dolphin.jpg (1024 x 800)

    I need to reduce this to 100 x 100

    How do i use what you have developed to do that.

    You help would be very useful.

    thanks a lot

    Deepesh

  • Excelent, after a long research, the exact thing I needed to present a small image when I have no photo to ilustrate my articles.

    Thank you

  • Have you checked out ImageSense for image resizing? It takes care of all the edge cases and you can easily convert formats (bmp to gif to jpg to png, etc) as part of your filter rules when doing a resize event. It also supports cropping, rotating, and color filters while preserving transparency.

  • Damn, thanks for the help, that was a simple solution that you explained clearly. Putting it to use now.

  • By the way... let me guess that in the picture you are the one who is wearing jean jacket !!!
    Thanks!

  • Isn't there any easier method? Like isn't there simple software that allows you to resize and not have resolution degradation? I am using an expensive service ShutterStock to buy my photos. I have been downloading them extra large thinking that when I "reduced" then they surely would not loose quailty. I did this size reduction in Microsoft Office Picture Manager and the result is unbelievably bad. What's going on?

  • This is the method to use if you need image resizing functionality in your applications. Using command-line or COM-based external apps for this will be overkill in web context. In web systems with many users I am sure you don't want to offer image resizing functionality to users the manual way :)

  • how to do it without fileupload control ? like i have the path of the file ( which is gonna be resize) so its not converting into stream but this function need stream how to resize in that case ?
    thanks

  • Hi,
    it would be nice if you also show how to define quality of final picture (jpeg compression).

    Also you should use "using" or "try-finally>Dispose" approach instead of calling "Dispose" at the end of methods. It can cause memory leaks when more larger files are processed.

  • You surely deserve a round of applause for your post and more particularly, your blog in common. Really top quality material

  • This code was very use full but while using a image path which been already file upload and it's in folder. while download only i need to resize in this at stream getting error

  • protected void btndwn_Click(object sender, EventArgs e)
    {
    string targetPath = Server.MapPath("Images/");

    //At this place getting error(
    Stream strm = targetPath.InputStream;
    )

    var targetFile = targetPath;

    GenerateThumbnails(0.5, strm, targetFile);
    }
    private void GenerateThumbnails(double scaleFactor, Stream sourcePath, string targetPath)
    {
    using (var image = Image.FromStream(sourcePath))
    {
    var newWidth = (int)(image.Width * scaleFactor);
    var newHeight = (int)(image.Height * scaleFactor);
    var thumbnailImg = new Bitmap(newWidth, newHeight);
    var thumbGraph = Graphics.FromImage(thumbnailImg);
    thumbGraph.CompositingQuality = CompositingQuality.HighQuality;
    thumbGraph.SmoothingMode = SmoothingMode.HighQuality;
    thumbGraph.InterpolationMode = InterpolationMode.HighQualityBicubic;
    var imageRectangle = new Rectangle(0, 0, newWidth, newHeight);
    thumbGraph.DrawImage(image, imageRectangle);
    thumbnailImg.Save(targetPath, image.RawFormat);
    }

  • Was using the GetThumbnailImage method for thumbnails on a website with poor results. This posting gave me exactly what I needed to decide how to improve it and get it done right.

  • To resize videos you need some library that can handle videos. Before going with commercial ones take a look at NuGet packages that work with ffmpeg.

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