Using a canvas much larger than the screen

I'm trying to draw a 2D contour plot of some data on Android and I'm wondering what the best approach would be to draw those. The whole datasets can be relatively large (2k * 2k points) and zooming and moving inside the plot should be very fast. Most of the time only a small part of the data will be drawn as the user has zoomed in on the data.

My idea now would be to draw the whole plot onto a large canvas, but clip it to the portion visible on the screen, so that on开发者_StackOverflow社区ly that part would be really drawn in the end. I find the 2D drawing API of Android somewhat confusing and I'm not sure if this is really a feasible approach and how I would then go about executing it.

So my questions are:

• Is it a good idea to draw onto a canvas much larger than the screen and use clipping to display only the relevant part?
• How would I create a larger canvas and how would I select which parts should be drawn?

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You should start the other way around. Instead of creating a huge canvas you should detect what part of your plot you need to draw and draw only that.

So basically you need some navigation/scrolling and you need to keep the offset from the starting point in memory to calculate where you are. Using the offset you can easily zoom in and out because you just need to scale the plot to the screen.

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Is it a good idea to draw onto a canvas much larger than the screen and use clipping to display only the relevant part?

A better question might be, do you have any other options. Some might argue that this is a bad idea since your going to keep memory in use when it isn't relevant to whats happening on the UI. However, from my experiences with the Canvas, I think you'll find this should work out just fine. Now, if you are trying to keep "5 square miles" of canvas in memory your definitely going to have to find a better way to manage it.

How would I create a larger canvas and how would I select which parts should be drawn?

I would expect that you will be creating your own "scrolling" method when the user touches the screen via overriding the onTouchEvent method. Basically your going to need to keep track of a starting point X and Y and just track that value as you move the Canvas on screen. In order to move the Canvas there are a number of built in's like translate and scale that you can use to both move the Canvas in X and Y as well as scale it when the user zooms in or out.

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I don't think that is a good idea to draw your 2D contour plot on a big bitmap because you need a vector type graphics to zoom in and out in order to keep it sharp. Only pictures are good to scale down but graphs will lose thin lines or come out deformed when scaled down in bitmaps.

The proper way is to do it all mathematically and to calculate which part of the graph should be drawn for required position and zoom. Using anti_alias paint for lines and text, the graph would always come out sharp and good... When the user zooms out, some items should not be drawn as they could not fit into the screen or would clutter it. So the graph would be always optimised for the zoom level...