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I'm making a little Tetris clone using some of the HTML 5 features, and I've come to a bit of a dilemma.

My tetrominos aren't composed of blocks -- they have shading and shadows that depend on the rotation and thus each is its own image. Which means I can't use the normal "redraw everything" every time the game state changes, because that would mean keeping track of all the piece drop history and "replaying" it whenever I want to clear a line.

So my options are:

1) Create two canvas elements stacked upon each other. The one on the back is the "board" image. The one on top is the pieces canvas, where I can slice and rearrange things easily. This feels very hackish though, as I wanted to keep everything in one canvas element.

2) Create a "drawing" context and a "buffer" context. In the buffer context, I have all of the pieces and can manipulate it freely, and when it's time to draw the buffer to the screen, I getImageData, loop through all of the pixels, and composite them onto the board image. I then write this into the "drawing" context with putImageData. This hassle occurs because I can't find a way to take an ImageData structure and composite it onto a canvas element (you can do that with ImageElements using drawImage, but drawImage won't take an ImageData structure).

Neither solution seems very elegant to me.

Thanks

Nick
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1 Answers1

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There's nothing 'hackish' about layering canvases.

Similar strategies are performed at several level in many systems from old coin-operated consoles to modern GPUs, passing though 80's 8bit home computers, and that's without mentioning classic hand-drawn cartoons (which were drawn on transparent acetate to use similar techniques).

in fact, even if you do other kinds of manipulation, it's quite likely you should do layering for lots of reasons.

Javier
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  • Consider using Easeljs. Easeljs caches your display list so that only branches that have been invalidated get re-renderred. All others are drawn as one bitmap (very fast); however, even with that feature, layering canvases is the best way to distribute the rendering load. – CharlesTWall3 Mar 29 '12 at 19:18