XNA - Merge sprites for better drawing performance?

Question

I read somewhere that when rendering a lot of 3D objects one could merge these to form a giant mesh so that only one draw call would be made. Therefore letting the, quote: "GPU do its magic", while the CPU is free for other calls than draw.

So to my question, would this be feasible to do in a 2D environment with performance in mind?

For example, say we have a simple tile-system and instead of making a draw call for each tile in view, one would instead merge all tiles to form a large sprite and then call a draw on it.

Any insight into the matter - either tips, links or whatnot - is greatly appreciated since I have no prior experience in graphics performance.

Edit: Sorry for my poor explanation. I am creating a tileengine for personal use and want it to be as versatile as possible. Therefore I want to optimize just in case I have to draw lots of tiles in the near future.

I do use a tilesheet but what I meant with the question is if merging all tiles that are to be drawn from the sheet into a new Texture2D will gain performance. For example:

We have 128x72 tiles to draw on the screen. Instead of looping through all tiles and calling draw for each tile to be drawn, we merge all tiles into a new sprite 1280x720 in size and draws it. This way the draw() method will only be called once per frame. My question was if this will improve performance as it would merging 3d objects into a single mesh when doing 3D.

Because the info I have gathered is that calling draw() is a performance hog and should be called as little as possible. Anyone to confirm or deny? :)

Answer 1

I don't have any experience with XNA and 3D, but I will give you some advice for 2D games. I spent some time creating a tile engine in XNA at the beginning of this year and wondered the same thing. I think the short answer here is yes, combining your tiles into a larger sprite is a good idea if you're worried about performance. However, there is a much longer answer if you're interested.

In general, when it comes to performance optimizations, the answer is almost always, "don't do it." If you're sure you need to optimize for performance, the next answer is almost always, "don't do it yet." Finally, if you do attempt to optimize for performance, the most important thing you can do is use benchmarks to gather precise measurements of the performance before and after the changes. Without that, you don't know if you're succeeding!

Now, related more to 2D games, I learned that I saw a lot better performance in my tile engine the less I switched textures. For example, let's say I have a grass tile and a gravel tile. If these are two separate textures in memory, and I draw a grass tile, then a gravel tile, then a grass tile to the screen, the GPU will load the grass texture, then switch it out to load the gravel texture, then switch the grass texture back in to draw another grass tile. This kills performance really quickly! The easiest way to get around this is to have a spritesheet, where you put your grass and gravel tiles into one texture, and just tell SpriteBatch to draw from a different area on the same texture each time.

Another thing to consider is how many tiles you're going to be drawing on the screen at once. I can't remember specifically, but I was drawing thousands of tiles at once (in a zoomed out view). I noticed that when I used bigger tiles and drew fewer of them, as you are suggesting in your question, that performance also improved. This wasn't as big of an improvement as what I described in the last paragraph though, and I would still encourage you to measure the performance changes resulting from different implementations. Also, if you're only drawing a dozen or few hundred tiles it may not be worth it to try to optimize that right now (see the 2nd paragraph).

Just so you know I'm not completely making this up, here's a link to a post from Shawn Hargreaves on texture swapping, spritesheets, etc. There are probably better posts on the XNA forums as well as Shawn Hargreaves' blog if you search on the topic.

http://forums.xna.com/forums/p/24254/131437.aspx#131437

Update:

Since you updated your question, let me update my post. I decided to just benchmark some samples to give you an idea of what the performance differences might be. In my Draw() function I have the following:

        GraphicsDevice.Clear(Color.CornflowerBlue);

        Stopwatch sw = new Stopwatch();
        sw.Start();

        spriteBatch.Begin();

#if !DEBUG
        spriteBatch.Draw(tex, new Rectangle(0, 0, 
                         GraphicsDevice.Viewport.Width,
                         GraphicsDevice.Viewport.Height), 
                         Color.White);            
#else
        for (int i = 0; i < 128; i++)
            for (int j = 0; j < 72; j++)
            {
                Rectangle r = new Rectangle(i * 10, j * 10, 10, 10);
                spriteBatch.Draw(tex, r, r, Color.White);
            }
#endif
        spriteBatch.End();

        sw.Stop();

        if (draws > 60)
        {
            numTicks += sw.ElapsedTicks;
        }
        draws++;

        if (draws % 100 == 0)
            Console.WriteLine("avg ticks: " + numTicks / (draws - 60));

        base.Draw(gameTime);

Just drop the exclamation point in the "#if !DEBUG" statement to switch between the two methods. I skipped the first 60 draws because they included some initial setup (not really sure what) and were skewing the averages. I downloaded one 1280x720 image, and for the top test case I just drew it once. For the bottom test case I drew the one source image in tiles, 128x72 like you asked about in your question. Here are the results.

Drawing one image:

avg ticks: 68566
avg ticks: 73668
avg ticks: 82659
avg ticks: 81654
avg ticks: 81104
avg ticks: 84664
avg ticks: 86626
avg ticks: 88211
avg ticks: 87677
avg ticks: 86694
avg ticks: 86713
avg ticks: 88116
avg ticks: 89380
avg ticks: 92158

Drawing 128x72 tiles:

avg ticks: 7902592
avg ticks: 8052819
avg ticks: 8012696
avg ticks: 8008819
avg ticks: 7985545
avg ticks: 8028217
avg ticks: 8046837
avg ticks: 8291755
avg ticks: 8309384
avg ticks: 8336120
avg ticks: 8320260
avg ticks: 8322150
avg ticks: 8381845
avg ticks: 8364629

As you can see, there's a couple order of magnitude difference there, so it's pretty significant. It's pretty simple to test this kind of thing, and I'd recommend you run your own benchmarks for your specific setup to take into account something I might've missed.

Answer 2

The less the Graphics Card has to switch textures the better the performance. You can sort by Texture in the SpriteBatch.Begin() method if you like so that the Graphics Card will switch textures as little as possible.

I put my tiles / sprites into sheets when it makes sense. For instance one character may be in a sheet and one layer of tiles may be in the same sheet. This has worked pretty well thus far.

I would say that don't optimize until you have to though. If your games runs fast enough as is why bother with optimization.