How to display images without taking up huge amounts of RAM

Question

I'm working on a silverlight project where users get to create their own Collages.

The problem

When loading a bunch of images by using the BitmapImage class, Silverlight hogs up huge unreasonable amounts of RAM. 150 pictures where single ones fill up at most 4,5mb takes up about 1,6GB of RAM--thus ending up throwing memory exceptions.

I'm loading them through streams, since the user selects their own photos.

What I'm looking for

A class, method or some process to eliminate the huge amount of RAM being sucked up. Speed is an issue, so I don't want to be converting between images formats or anything like that. A fast resizing solution might work.

I've tried using a WriteableBitmap to render the images into, but I find this method forces me to reinvent the wheel when it comes to drag/drop and other things I want users to be able to do with the images.

Answer 1

What I would try is to take load each stream and resize it to a thumbnail (say, 640x480) before loading the next one. Then let the user work with the smaller images. Once you're ready to generate the PDF, reload the JPEGs from the original streams one at a time, disposing of each bitmap before loading the next.

Answer 2

I'm guessing you're doing something liek this:

Bitmap bitmap = new Bitmap (filename of jpeg);

and then doing:

OnPaint (...)
{
   Graphics g = ....;
   g.DrawImage (bitmap, ...);
}

This will be resizing the huge JPEG image to the size shown on screen every time you draw it. I'm guessing your JPEG is about 2500x2000 pixels in size. When you load a JPEG into a Bitmap, the bitmap loading code uncompresses the data and stores it either as RGB data in a format that will be easy to render (i.e. in the same pixel format as the display) or as a thing known as a Device Independant Bitmap (aka DIBitmap). These bitmaps require more RAM to store than a compressed JPEG.

Your current implementation is already doing format conversion and resizing, but doing it in an innefficent way, i.e. resizing a huge image down to screen size every time it's rendered.

Ideally, you want to load the image and scale it down. .Net has a system to do this:

Bitmap bitmap = new Bitmap (filename of JPEG);
Bitmap thumbnail = bitmap.GetThumbnailImage (width, height, ....);
bitmap.Dispose (); // this releases all the unmanged resources and makes the bitmap unusable - you may have been missing this step
bitmap = null; // let the GC know the object is no longer needed

where width and height are the size of the required thumbnail. Now, this might produce images that don't look as good as you might want them to (but it will use any embedded thumbnail data if it's present so it'll be faster), in which case, do a bitmap->bitmap resize.

When you create the PDF file, you'll need to reload the JPEG data, but from a user's point of view, that's OK. I'm sure the user won't mind waiting a short while to export the data to a PDF so long as you have some feedback to let the user know it's being worked on. You can also do this in a background thread and let the user work on another collage.