Question

I’m planing to create a simple game using the HTML5 <canvas> tag and compile the code into a native application using Phonegap, but the problem is that canvas use coordinates that are not relative to the size of it, so 20,20 on a 960×640 screen is different on a 480×800 one.

So I want to know how to work with a <canvas> (which will be in fullscreen) on different screen sizes.

Answer 1

So I want to know how to work with a (which will be in fullscreen) on different screen sizes.

This is a common problem that has a pretty easy resolution. Often this is done by separating hard canvas coordinates from what is sometimes called “model” coordinates.

It really depends on how your code is organized, but I assume the game has some height and width to the world that takes up most or all of the screen. The two aspect ratios of the screens you are targeting are 1.5 and 1.666, so you’ll want to cater to to the smaller one

So you’ll really want to do your game in a set of “model” coordinates that have no bearing on the screen or canvas sizes. Since you are only targeting two screen sizes, your model coordinates can perhaps be 960×640 since that is the smaller of the two aspect ratios. It doesn’t have to be. It could be 100×50 for your model coordinates instead. But this example we’ll use 960×640 as our model coordinates.

Internally, you never use anything but these model coordinates. You never ever think in any other coordinates when making your game.

When the screen size is 960×640 you won’t have to change anything at all since its a 1:1 mapping, which is convenient.

Then when the screen size is actually 800×480, when it comes time to draw to the screen, you’ll want to translate all of the model coordinates by (3/4), so the game will be made and internally use 960×480, but it will be drawn in the area of (720×480). You’ll also want to take any mouse or touch input and multiply it by (4/3) to turn the screen coordinates into model coordinates.

This translation can be as easy as calling ctx.scale(3/4, 3/4) before you draw everything.

So both platforms will have code that is all written assuming the game is a size of 960×640. The only time that model coordinates become screen coordiantes is when drawing to the canvas (which is a different size) and converting canvas mouse/touch coordinates to model coords.

If that seems confusing to you I can try and make a sample.