How can I get a smooth text crawl using Flex?

Question

I'm working on a standalone Flash application (written using Flex 3/ActionScript 3) that features a text crawl, like what you might see at the bottom of your TV when watching a cable news channel; it's a long narrow box that text moves across from right to left.

I've implemented it by creating a Label element, populating it with text, and then moving it using a mx:Move object with a Linear.easeNone easing function. It works, but it has ample room for improvement. It looks a bit jerky, and tends to have a fair amount of "tearing" (the top and bottom halves of the text sometimes fall out of sync).

I tried throwing math at the problem to get the crawl's movement rate synced with the monitor's refresh rate, but that was a bust. I found out the hard way that the app's frame rate jumps around too much; the "optimized" crawl varied between looking silky smooth and like it had epilepsy.

Is there anything else folks would recommend I try to smooth this thing out? Is there some alternate design you'd recommend I try?

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Edit: Some context: the crawl is part of a digital signage application (played from a standalone Flash projector -- no web browser) that does stuff elsewhere on the screen, including video playback and rendering text and images. It definitely gets choppier during video playback, but it's never as smooth as I'd like it to be.

Answer 1

There are two potential solutions to this problem, but both have caveats, the first because of your use of Flex and a standalone projector, the second because it is a mitigator, not a complete solution.

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Hardware Acceleration

When publishing your file, you can attempt to have Flash utilize hardware acceleration to alleviate the vertical refresh issue you are running into that is causing tearing. Sadly, Flex Builder 3 is incapable of enabling this setting at the SWF (projector) level (Link to bug). This has yet to be resolved and has been pushed from 4.0 to 4.1 to 4.x... If and when it is resolved, it will likely be a compiler argument in the project settings of Flash Builder 4.

You may be able to determine if this solution works for you by outputting your projector as a standard SWF and embedding it on an HTML document with the wmode set to "direct" or "gpu". Sadly, if it does (it should), you can't use it right now anyway. If you have Flash Builder 4, certain projects are capable of making round trips between FB4 and Flash Professional CS5, though I am not sure what the criteria for that is (my current AIR project has all the project modification menu options grayed out). If you do manage to get your project into Flash, you can enable hardware acceleration in the Publish Settings of the project (File->Publish Settings->Flash tab->Hardware Acceleration option in CS5).

This method is almost a certain solution for your problem, though it has two issues, one already highlighted above, and (for people publishing for the web) that by utilizing direct or GPU rendering on a webpage, you are unable to layer any DOM elements on top of flash.

direct: This mode tries to use the fastest path to screen, or direct path if you will. In most cases it will ignore whatever the browser would want to do to have things like overlapping HTML menus or such work. A typical use case for this mode is video playback. On Windows this mode is using DirectDraw or Direct3D on Vista, on OSX and Linux we are using OpenGL. Fidelity should not be affected when you use this mode.

gpu: This is fully fledged compositing (+some extras) using some functionality of the graphics card. Think of it being similar to what OSX and Vista do for their desktop managers, the content of windows (in flash language that means movie clips) is still rendered using software, but the result is composited using hardware. When possible we also scale video natively in the card. More and more parts of our software rasterizer might move to the GPU over the next few Flash Player versions, this is just a start. On Windows this mode uses Direct3D, on OSX and Linux we are using OpenGL.

**Source*

Direct is the ideal option for this situation, as you can actually have performance degredation with "gpu" as well as visual differences from graphics card to graphics card.

Lower your framerate

The Flash player will continue to play video at its native refresh rate independent of the rest of your project as long as you keep the framerate at or above approximately 2FPS (though I suggest 5FPS minimum). You won't want to run that low for this example, but you are able to lower the framerate of the entire scene without impacting video performance. The closer your framerate is to the screen refresh rate, the more apt you are to actually create the tearing effect unless you are able to absolutely sync with the monitor's refresh rate, which you probably cannot do without the above... Hardware Acceleration.

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This problem has existed in the Flash Player for as long as it has been able to move objects horizontally. What happens is that Flash updates a buffered snapshot of the running animation at the same time that the screen is refreshing. If the buffered snapshot changes partway through a screen refresh, you get a tear. This is why lowering the framerate actually reduces the amount of tearing, you are refreshing the buffer less frequently.

Answer 2

As @Tegeril mentioned, using Flex is one of the reasons. Flex is a pretty heavy framework and it does a lot of things behind the scenes. If you're familiar with the life cycle of a component(especially invalidating properties, invalidating the display list, etc.).

As a few minor things that might improve performance:

• try to keep a simple display list. If you know the app will always be displayed at one size, then flex won't waste time traversing the display list/tree up to the top and back for measurements. Also, try to use a Canvas. I know, it's not very clean, but since it uses absolute values and doesn't check with the 'parents' much, it should be faster than other containers(like HBox,VBox, etc.)
• try to display the video at it's full size(make sure the encoded video dimensions are right so there be any CPU cycles on resizing video

Ok, this was Flex stuff.

It might be very handy to read sencular's article on Asynchronous ActionScript Execution which explains how Flash Player handles updates and renders.
_{(source: senocular.com)}

Frames both execute ActionScript and render the screen

_{(source: senocular.com)}

ActionScript taking a long time to complete delays rendering

I imagine the jerkiness is related to this. Also, I'm guessing you might get moments of smooth movement then sudden halts, every now and then, when Flash Player catches it's breath(Garbage Collector cleans up)

Victor Drâmbă article on “Multithreading” in Actionscript might also be useful.

Soo, to recap:

• use Profiler or something and see if the Flex framework is slowing you down, or where the 'bottleneck' is
• improve as much as you can on that side then check if it's how Flash Player handles all the actionscript('elastic' frames)

If the bottleneck comes from the Flex framework, worst case, you can try to minimise the number of components that traverse the display list, and use pure actionscript for the other things(as @PatrickS suggested, use TweenLite, etc.)

If it helps, try to preload data(fetch rss feed and all that) at the start, and when you've got most of the important bits that don't require 'refreshes'/loads frequently, display the app. You will use more memory, but will have more cpu cycles to spare for other tasks. Also, if it's display objects that are the 'bottleneck' and there's plenty of them, check if you can reuse them using Object Pools.

HTH