EXPLOSIONS, BLITTING, AND OPTIMIZATION in Font

Creator Code-128 in Font EXPLOSIONS, BLITTING, AND OPTIMIZATION

A major problem with all the particle effects we ve looked at so far is that they re slow. As soon as you have more than a few hundred particles flying around the stage, the frame rate takes a dramatic nosedive. These examples have been an essential step in the learning process, but for almost any practical or professional game project, they re not usable. Yes, I mean they re useless! The particles we have worked with are composed of Sprite or MovieClip objects. These objects are very convenient to work with for several reasons: They re simple to create and manage. They can be easily rotated, faded, and scaled. It s easy to change the stacking order (which objects appear in front of or behind other objects). But that convenience comes with a big processing overhead. If there s any way we can display things on the stage without sprites or movie clips, we can win back bucket-loads of CPU power. It turns out, there is a way.

What s bit-block transfer It sounds like the final stage of some devious human/machine cybernetics experiment, or perhaps even more distressingly, a particularly uninspired children s educational board game. Well, my dears, we ve arrived at the obligatory section of the book where I m going to ask you to brew yourself a pot of cinnamon-clove rooibos tea, do a few sun salutations, and then settle down under a shady tree in a secluded forest glen, with nothing but the fluttering of butterfly wings and the whir of your laptop s fan to disturb you. I need you to have your senses at their sharpest as I introduce a topic that is not only essential for creating usable particle-explosion effects, but also a cornerstone game-design technique. Yes, it s bit-block transfer! All the examples in this chapter have made use of a funky little method called copyPixels. It s used to copy pixels from one bitmap to another. La-di-da! But this seemingly mundane task holds a deep, dark secret that if you use it to your advantage, can completely revolutionize what your Flash games are capable of doing. Adobe s AS3.0 documentation, as useful as it is, is not known for its gripping narrative style. And there s certainly no trace of the personality of whoever wrote it except for one, telling moment. If you look up the entry for copyPixels, you ll find this:

Whoever the anonymous author was, he or she invoked Herculean restraint in writing that sentence. The only trace of what the author is attempting to imply is the word fast, which I m sure was added with immense pride by Adobe s engineering team. Because, in reality, that sentence should read like this:

[copyPixels] provides a staggeringly, Earth-shatteringly, hair-raisingly fast routine to perform pixel manipulation between images that will blow your mind, man!

How fast is it Depending on how you cut the cake, you can use it to move things around the stage about three to five times faster than with Sprite or MovieClip objects. Five times more processing power at your disposal means five times more things moving around the stage, or five times more power to do other important processing in your game.

A series of comprehensive tests by Jeff and Steve Fulton, authors of The Essential Guide to Flash Games (friends of ED, 2010), showed that in certain circumstances, using copyPixels can actually be up to 20 times faster. Performance is notoriously dependent on context, so you ll need to run some tests in your own projects to find out exactly what kind of performance saving you can get from bit-block transfer.

Not only that, but using copyPixels to move objects around the stage uses less code than moving around sprites or movie clips, and it s delightfully elegant to implement. This leads to only one conclusion: Always use copyPixels to move objects whenever you can. Use Sprite or MovieClip objects if you need the extra icing sugar they provide often you won t. How then can you harness the speed of copyPixels This is where the technique of bit-block transfer comes in. It s an ancient and venerable video-game animation technique that has been around since the days of Pong. It s used to copy pixels from one bitmap into a new place in a second bitmap. Bit-block transfer isn t a phrase that rolls easily off the tongue, so it s more affectionately known as blit or blitting. For a basic blit, you start with a bitmap image that contains all the objects you want to use in your game. Yes, that s right: all the objects in one big bitmap. Arrange them in a grid formation. These can be characters, elements of the environment, rendered text, or whatever you like. In a nutshell, the image should contain all of your game graphics. The single bitmap that they re contained in is called a tile sheet. The bitmap file can be a PNG, JPEG, or GIF. The graphics are organized in a grid layout, and should be contained in grid rectangles or squares of the same size. You can use any bitmap image-editing software you like, such as Photoshop, Fireworks, or GIMP.