Android scrolling background of bitmap tiles

Question

I'm tried to determine the "best" way to scroll a background comprised of tiled Bitmaps on an Android SurfaceView. I've actually been successful in doing so, but wanted to determine if there is a more efficient technique, or if my technique might not work on all Android phones.

Basically, I create a new, mutable Bitmap to be slightly larger than the dimensions of my SurfaceView. Specifically, my Bitmap accomodates an extra line of tiles on the top, bottom, left, and right. I create a canvas around my new bitmap, and draw my bitmap tiles to it. Then, I can scroll up to a tile in any direction simply by drawing a "Surfaceview-sized" subset of my background Bitmap to the SurfaceHolder's canvas.

My questions are:

1. Is there a better bit blit technique than drawing a background bitmap to the canvas of my SurfaceHolder?

2. What is the best course of action when I scroll to the edge of my background bitmap, and wish to shift the map one tile length? As I see it, my options are to:

a. Redraw all the tiles in my background individually, shifted a tile length in one direction. (This strikes me as being inefficient, as it would entail many small Bitmap draws).

b. Simply make the background bitmap so large that it will encompass the entire scrolling world. (This could require an extremely large bitmap, yet it would only need to be created once.)

c. Copy the background bitmap, draw it onto itself but shifted a tile length in the direction we are scrolling, and draw the newly revealed row or column of tiles with a few individual bitmap draws. (Here I am making the assumption that one large bitmap draw is more efficient than multiple small ones covering the same expanse.)

Thank you for reading all this, and I would be most grateful for any advice.

Answer 1

I am using your original method to draw a perspective scrolling background. I came up with this idea entirely by accident a few days ago while messing around with an easy technique to do a perspective scrolling star field simulation. The app can be found here: Aurora2D.apk

Just tilt your device or shake it to make the background scroll (excuse the 2 bouncing sprites - they are there to help me with an efficient method to display trails). Please let me know if you find a better way to do it, since I have coded several different methods over the years and this one seems to be superior. Simply mail me if you want to compare code.

Answer 2

I originally used a similar technique to you in my 'Box Fox' platformer game and RTS, but found it caused quite noticeable delays if you scroll enough that the bitmap needs to be redrawn.

My current method these games is similar to your Option C. I draw my tiled map layers onto a grid of big bitmaps (about 7x7) taking up an area larger than the screen. When the user scrolls onto the edge of this grid, I shift all the bitmaps in the grid over (moving the end bitmaps to the front), change the offset of grid, and then just redraw the new edge.

I'm not quite sure which is faster with software rendering (your Option C or my current method). I think my method maybe faster if you ever change to OpenGL rendering as you wouldn't have to upload as much texture data to the graphics card as the user scrolls.

I wouldn't recommend Option A because, as you suggest, the hundreds small bitmap draws for a tiled map kills performance, and it gets pretty bad with larger screens. Option B may not even be possible with many devices, as it's quite easy to get a 'bitmap size exceeds VM budget' error as the heap space limit is set quite low on many phones.

Also if you don't need transparency on your map/background try to use RGB_565 bitmaps, as it's quite a lot faster to draw in software, and uses up less memory.

By the way, I get capped at 60fps on both my phone and 10" tablet in my RTS with the method above, rendered in software, and can scroll across the map smoothly. So you can definitely get some decent speed out of the android software renderer. I have a 2D OpenGL wrapper built for my game but haven't yet needed to switch to it.

Answer 3

I think one efficent way to do this would be to use canvas.translate.

On the first drawing the entire canvas would have to be filled with tiles. New android phones can do this easily and quickly.

When the backround is scrolled I would use canvas.translate(scrollX, scrollY), then I would draw individualy one by one tile to fill the gaps, BUT, I would use canvas.drawBitmap(tileImage[i], fromRect, toRect, null) which would only draw the parts of the tiles that are needed to be shown, by setting fromRect and toRect to correspond to scrollX and scrollY.

So all would be done by mathematics and no new bitmaps would be created for the background - save some memory.

EDIT: However there is a problem using canvas.translate with surfaceView, because it is double buffered and canvas.translate will translate only one buffer but not the second one at the same time, so this alternating of buffers would have to be taken into account when depending on surfaceView to preserve the drawn image.

Answer 4

My solution in a mapping app relies on a 2 level cache, first tile objects are created with a bitmap and a position, these are either stored on disk or in a Vector (synching is important for me, multithreaded HTTP comms all over the place).

When I need to draw the background I detect the visible area and get a list of all the tiles I need (this is heavily optimised as it gets called so often) then either pull the tiles from memory or load from disk. I get very reasonable performance even on slightly older phones and nice smooth scrolling with no hiccups.

As a caveat, I allow tiles not to be ready and swap them with a loading image, I don't know if this would work for you, but if you have all the tiles loaded in the APK you should be fine.