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Tag Archives: BitmapData

Image caching with Flex 3

As always I will start with my examples because I know most people want to see a demo before reading about it. Both of these examples are identical except for one line of code which flags image caching on or off. I have added stats.as so you can see the memory usage as you load more images. You will notice that when using the example with image caching you can load hundreds of images with little memory impact and it is much faster than the example with out caching. Also if you do not believe stats.as open up your task manager and view the memory usage of your browser to confirm that ImageCache is really working.
View source is enabled so you can see whats going on.

App NOT using cache
App using cache

I have seen several Flex 3 image caching solutions out on the web but most of them seemed to subclass SWFLoader or Image. In my opinion the Image class should know nothing about caching, it should just know about displaying the image. Why add extra functionality to the Image class? Why not have something else manage the caching? So I created ImageCache which is a utility class that handles caching images which can then be used as the source for any Image component.

The reason I like this approach is that it leaves the Image class alone and if you want to cache something you do not need to swap all the Images in your app with some subclass. You only need to change the way you set the Images source property.

I thought the browser cached images?

You may think that you don’t need to cache your images in Flex because your browser is already doing that. While it is true that your browser is caching the images that your Flex app requests it probably works a little different then you think. For example the first time you load an image you might notice that it takes a second, but if you load that same url again it shows up right away. This must mean that it is using a cached version of the image right? Well thats partly correct your browser is giving you a cached version of the image BUT, your Flex app is creating a whole new instance of that image data. So by default you get a speed gain with your browsers cache but what you are not getting is any memory gains. Each time you load that image your Flex app’s memory usage will go up by the size of that image. This is because each Image component keeps a separate BitmapData for its own use. In some cases this is good but most of the time it is not necessary and can be wasteful.

If we cache the images in the app it self we can achieve even faster loading speeds because we do not even need to ask the browser for the image. Second and even better, we can reuse the same image over and over with almost no memory overhead. This is done by sharing the same BitmapData between multiple Image components.

How it works

ImageCache works by keeping a mapping between URL and BitmapData. When you use ImageCache to cache an image it will load the image and then store the BitmapData for that image. Then when you want to set an Image component’s source to that image ImageCache will create a new Bitmap using a reference to the stored BitmapData. This means that there is almost no memory overhead and the image will load almost instantly because we are just referencing the BitmapData instead of creating a whole new copy of it. One thing to note is that because of the BitmapData sharing if you change the underlaying BitmapData of one of the Images all of the Images referencing that BitmapData will also change. This may or may not be good for you depending on how your app works.

ImageCache currently has 4 public methods:

public function setImageSource(image:Image, source:Object):void
public function cacheImage(source:Object):void
public function flushImage(source:Object):void
public function loadIntoCache(source:Object):void

The setImageSource method allows you to set an Image’s source using the cache. If the url is not cached already this method will cache the image and set the Image component’s source. If the image is already cached it will just set the Image component’s source.

Next the cacheImage method will simply cache an image with out setting any component’s source. This is useful for pre-caching images so they can be used later on with out having to load them.

If you want to flush an image from the cache you can use the flushImage method to do so. This method removes an image from the cache meaning that the next time it is requested it will ask the browser.

Lastly the flush method will clear all images in the cache.

When you use the setImageSource method you do not have to worry about timing or whether or not the image has been cached yet because ImageCache already accounts for that. It will keep a list of Image components that want a certain source and once it has finished caching it will go back and set it on all the Images that were waiting.

One more thing. I require Image components but that could easily be swapped out for SWFLoader or whatever class you wanted. This is just a quick example to show the benefits of using caching in your app. The same techniques could also be used to cache any type of file your app might be loading .swf, .pdf, .xml and so on.

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Maintaining transparency with BitmapData

This took me a while to figure out but it was really simple once I read the docs.

BitmapData.draw example

You want to use the BitmapData class’s draw method to get a “copy” of some thing in ActionScript. You simply create a new BitmapData and then use the draw method and all is well right? Not quite. If you are using the draw method on something that has transparentcy, like a .png image, you might notice that now all of your transparent pixels are white. This is not a bug it is simply because you have not properly set up your BitmapData. If you take a look at the docs BitmapData’s constructor takes a 4th optional parameter called fillColor. The fillColor is a unsigned int that defaults to 0xFFFFFFFF if you do not pass anything in. At first glance you might look at that default value and think hey there are two extra F’s in there, but that is indeed a correct value. The fillColor param is a 32-bit ARGB color value. This mean the first two F’s specify the alpha, then the next two the red, then green, and then blue. This color is used to fill the bitmaps image area and because it is set to 0xFFFFFFFF (solid white) by default you create an all white BitmapData.

Then when you call draw you are drawing on top of the already white BitmapData with the image that has transparency. It is no wonder all the transparent pixels turned white you are really just seeing through the image to the white background. Don’t worry this is very easy to fix just change the default fill color to something transparent like so:

var transBMD:BitmapData = new BitmapData(w, h, true, 0x00ffffff);

You can see here we set the fillColor to 0x00ffffff or white with alpha of 0, this means the BitmapData will have no background color and when we do the draw we will keep our transparency.

Here is a more complete example of the code.

var transBMD:BitmapData = new BitmapData(origImage.content.width * 0.5
    origImage.content.height * 0.5,
    true, 0x00ffffff); //Note the first two 0's mean the alpha level is 0

var trans:Matrix = new Matrix(); //a transform matrix
trans.scale(0.5, 0.5); //scale the image
transBMD.draw(origImage, trans); //draw original image but scale it
var img:Image = new Image();
img.source = new Bitmap(transBMD); //This image will be 50% smaller and still have the transparency

I should also note that if you use BitmapData.copyPixels it will preserve the transparency and is much faster than using draw, but does not allow for stretching, rotating, or adding color effects. If you are not going to be doing any of those copyPixels is a better choice because of it’s speed.

Here is a link to a live Flex example with view source enabled that shows how this all works.
BitmapData.draw example