Speeding up image loading in WPF using thumbnails
Technorati Tags: wpf, thumbnails, image, performance, slow, BitmapImage
During a recent WPF session I needed to build a ListBox that showed a bunch of images loaded from an arbitrary directory. Thanks to WPF's data binding, this was trivial - I just needed to get a collection of objects that each had a property pointing to the full path of the image, and WPF would take care of all the loading/displaying. Something like this:
1: <ListView ItemsSource="{Binding}">
2: <ListView.ItemTemplate>
3: <DataTemplate>
4: <Image Source="{Binding Path=FullPath}" />
5: </DataTemplate>
6: </ListView.ItemTemplate>
7: </ListView>
The assumption here is that the DataContext for this window is set to a collection of "Photo" objects. The Photo class has a member called "FullPath" which is just a string with the full path of the photo on disk - this is what the Image.Source member expects.
This worked, but it didn't take long to see a major issue: With today's cameras, loading multiple 5+ megapixel images could take a while (not to mention the RAM requirements).
After a little digging, I found a solution. There exists a feature in the BitmapImage class that allows you to load an image but tell it to only load a thumbnail. To use this, you have to step out of the shrink-wrapped data binding world and insert a converter into the equation. Basically, this converter will take the above string with the full image path, it will load the image (as a thumbnail), and pass it back into the Image.Source parameter as a BitmapImage, which it's happy to consume.
First, let's look at this converter's code and how it loads the thumbnail:
1: public class UriToBitmapConverter : IValueConverter
2: {
3: public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
4: {
5: BitmapImage bi = new BitmapImage();
6: bi.BeginInit();
7: bi.DecodePixelWidth = 100;
8: bi.CacheOption = BitmapCacheOption.OnLoad;
9: bi.UriSource = new Uri( value.ToString() );
10: bi.EndInit();
11: return bi;
12: }
13:
14: public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
15: {
16: throw new Exception("The method or operation is not implemented.");
17: }
18: }
Notice line #7. That's the magic line which tells it how big of a thumbnail to load. The smaller the number, the quicker the load, the lower the quality. Notice also line #8 - this is there to force the image file to be closed after it's loaded. Without that, I found that my app couldn't write back to the image file since the ListBox still had it open.
Next, let's look at the XAML change to insert this converter into the mix. You'll need to create a resource for it:
1: <Window.Resources>
2: <local:UriToBitmapConverter x:Key="UriToBitmapConverter" />
3: </Window.Resources>
The "local:" namespace directive on line #2 is one I'd made sure to add to my main "Window" declaration, like this (line #4):
1: <Window x:Class="MyClass.Demo"
2: xmlns="https://schemas.microsoft.com/winfx/2006/xaml/presentation"
3: xmlns:x="https://schemas.microsoft.com/winfx/2006/xaml"
4: xmlns:local="clr-namespace:MyClass"
5: Title="Demo" Height="300" Width="300">
Lastly, use this new resource in the Image element so that FullPath (a string) gets pushed through the converter before Image.Source gets it:
1: <Image Source="{Binding Path=FullPath, Converter={StaticResource UriToBitmapConverter}}" />
That's it. Your images will now load very quickly. Tweak the thumbnail size to vary the speed versus quality.
Avi
Comments
Anonymous
February 23, 2008
The comment has been removedAnonymous
February 26, 2008
Hi, I'm a newbie on WPF and wanting to create my own thumbnail viewer. Your code looks VERY interesting. Is it possible to get a sample project of your code above? Thanks! MorajodkAnonymous
September 07, 2008
another newbie, couldn't integrate it into a project and get it to work. the binding syntax to the class might help - or a sampleAnonymous
January 04, 2009
Aplikasi image browser yang ditulis sebelumnya awalnya mengandalkan loading image mentah-mentah ( fullAnonymous
May 20, 2009
Code works like a charm. I'm using it to display thumbnails of images in a particular folder. Since this view opens in a separate window it can take a pretty long time to load even just the thumbnails. Anyway you can do this for each thumbnail asynchronously? Maybe even giving a default image until the thumbnail is ready?Anonymous
July 23, 2009
Just what I was looking for! Thanks!Anonymous
September 15, 2009
Take a look at http://msdn.microsoft.com/en-us/library/ms748873.aspx to see how to do this direct in xaml.Anonymous
September 18, 2009
Hi there and many thanks for your article, really useful. I have the same question as Jon had, if there is any way to know the real dimensions before loading, or any other trick to achive the same as Jon. NiclasAnonymous
June 17, 2010
Thanks for the post, There is another easy way to make thumbnail is just use BitMapImage in XAML and specify the Pixel size <Image Stretch="Uniform" > <Image.Source> <BitmapImage DecodePixelHeight="200" CacheOption="" UriSource="{Binding Image}" /> </Image.Source> </Image>Anonymous
July 27, 2011
Great code and clean description .. thanksAnonymous
July 02, 2013
Grreat and useful code, but what do you think when you have to load 1000+ images?Anonymous
February 17, 2014
Thanks Yahoooooooooooooooooo! You solve a nightmare problem of my projects thanks if any one want to know how to implement this i am here waqasidrees@hotmail.com