In defense of MovieClips, Sprites and the display list (...the case against blitting?)
UPDATE: I have made some benchmarks, and the results are in! Original post:
“This is why I always run a raster pipeline. The DisplayObject API is a trick played on the gullible and the trusting…” — @bengarney
Ben Garney is a very smart game/engine developer with a background in C++ and known for developing the Torque engine, who has now moved into the Flash world with Pushbutton Engine and games like Social City. I come from the complete opposite direction. I’m a Flash guy with a background in quick turn-around, design driven creative Flash projects, who now focuses on game development. This difference in backgrounds, and dare I say *ideologies* is why I thought I should write a response to Ben’s recent post about blitting, and stick up for the display list a bit. If you haven’t read Ben’s article, I suggest you go do that, but for the sake of clarification, blitting is basically using copyPixels to manually draw your graphics to a single bitmap data object that you display via a single Bitmap object on the stage.
Firstly, there is a lot of truth in what Ben says. Here are the advantages I see for blitting:
- Renders faster if you have thousands of sprites.
- Although Ben didn't mention this, if you use the Flixel, Flashpunk or Pushbutton engines you get some other game engine functionality for free, although most of the benefits of e.g. Flixel are nothing to do with the fact that it uses blitting.
Now here are the advantages for using MovieClips, Sprites and Bitmaps in the native Flash display list:
- You get rotation, scaling, alpha, filters and colour effects for free. Want to add a glow or blur to something? It’s one line of code, and adds no filesize to your app. If you use blitting you have to somehow prerender all these effects, either as pngs that will increase the filesize of your game, or prerender at runtime, which will add a lot of complexity to your code and add to your development time, and in the cutthroat world of Flash games, time is money. If you combine the display list’s built in effects with an engine like TweenMax, you can make amazing animated effects in just a few lines of code.
- You can work easily with interactive designers. Most creative agencies have designers who are experts at using the Flash IDE to create beautiful layouts and animations that you can easily breathe life into with ActionScript. If you are working from flat PSDs for all your assets, you will spend a lot of time laying screens out in code, which is something you never really want to do. The reason Flixel etc are never used on advergames, e-learning games or other client projects is that there is no workflow for designers to add their branding magic.
- You get mouse events. You can easily create buttons and more complex interactive elements with complex hit areas. Now in a game it is very possible to use blitting on your gameplay area but still use the display list for all menus/gui, and I would definitely recommend this option over blitting *everything* like Flixel does. However, sometimes you want to have interactive menus and buttons inside the gameplay area, in which case it will be much easier for you to use the display list in your game rather than recreate all that functionality from scratch inside your blitted area.
- You can nest things inside other things. This is very useful for things like menus, but is also very a great way to create dynamic animations that can be altered at runtime. For example, if you have an animated character holding a weapon, if the weapon is a sprite inside the character sprite, you can easily swap out the contents of the weapon sprite to change weapons. If you are using blitting, this feature will be time consuming to recreate.
- It’s not as slow as people think. If you are just moving Bitmap objects around in the display list you will easily be able to have hundreds of sprites on screen, in a typical game. And as only displayObjects within the viewable stage area are drawn, you can actually have a game play area with thousands of sprites, so long as you can only view hundreds at a time. Turning off mouse interaction for the display objects (mouseChildren = false, mouseEnabled = false) makes everything run significantly faster too. I have a demo with 40,000 tiles scrolling on the display list smoothly at 120fps – as Flash only renders those tiles inside the viewable area of the stage. You can also use a hybrid blitting / display list approach by having a Bitmap objects on the display list which you animate by changing the value of the bitmap.bitmapData property. This is very fast and you still get all the advantages of the display list effects. The best of both worlds!
So ultimately it becomes a question of how much rendering speed ya needs versus how much development time ya gots (read that last sentence in the style of the Clint Eastwood). From what Ben says about Social City, it looks like a poster child for blitting – it’s a scene with thousands of sprites drawn at once (although Ben’s claim that his display list tests were running at less than 1 frame per second suggests to me that he had not correctly optimised by turning off mouse interaction). Either way, display list *will* always render slower for a thousand sprites. But pretty much every Flixel game I have seen would have run perfectly using the display list. Ben says that the display list is the darkside of the force. In equally mischievous tone, I propose that blitting is for people who don’t actually like Flash, and they see everything that makes it unique and interesting, like the excellent Flash IDE and the vector display list renderer, as a negative. I will get some benchmark demos of both approaches up soon. Of course display list will lose, but it won't be as bad as people think. Comments appreciated!