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    Improving a Sprite-Based Rendering Procedure

    Graphics and GPU Programming

    • Posted By j-jorge
    By considering a scene of the --work in progress-- game Plee the Bear, I will first describe in this article how much work is done during the default rendering procedure. Then it will be compared with an easy to implement improved procedure. And finally I will give you the pointers to an even better procedure. This final procedure has been used in Andy's Super Great Park.


    In the first versions of Plee the Bear we were not really worried about the speed of the rendering procedure, nor the speed of any other procedure. Keeping in mind that premature optimization is the root of all evil, we had to make things work before making them working fast. That was some years ago. Then the game has grown, we began to put a lot of stuff in the levels and finally the time of thinking about accelerating things did come. That is the subject of this article: how the rendering procedure evolved with the growing of the game. The initial procedure was as simple as possible. Elements are rendered from the background to the foreground, as is. Having something drawn on the screen was a sufficient result at this time. So, what amount of work does this procedure do? Let's see how many times each pixel of the screen is written in a given scene. We will use the very beginning of the first act of the forest of Plee the Bear, just when the player can start to control Plee:
    And we will apply the following color representation:
    Here is the result for the aforementioned scene:
    Not surprisingly, with three layers of rain plus the background, each pixel is written at least 4 times, most of them 5 or 6 times and some are written up to 9 times. And once the rain is gone, the range goes from 1 to 6 writings:
    An interesting thing in these two pictures is that even parts hidden by the middle ground decorations are rendered.

    Improving the rendering procedure

    The improvement we wanted to introduce then was to avoid rendering elements that will be hidden by other elements. The idea is to maintain a representation of the empty parts of the screen whilst considering the elements from the foreground toward the background. For each element there are two steps. First, if the element intersects the empty parts of the screen, we split it into sub-elements that will cover only the empty parts of the screen. Then, if the initial element is opaque, we update the emptiness of the screen. To keep things simple, we represent the parts of the screen with axis-aligned boxes. Elements are considered as opaque if there is no alpha transparency in the source image and if they are not rotated.
    Let's come back to the game to see how many times the pixels are written with this procedure. During the rain:
    Pixels are written from 2 to 8 times. Contrary to the original procedure, some of them are drawn 2 or 3 times. The number of pixels drawn more than 3 times has been greatly reduced. And after the rain:
    Here the range becomes 1-5 writings per pixels, most of them are written 1 or 2 times. Contrary to the original algorithm, we have more pixels written once than three times.

    The benchmark

    Finally, for all this work to be useful there must be an increase of the performance. That is: more frames rendered per second. To keep a uniform sequence of rendered items among the tests, we use a demo script that runs in the game. Here are the results:
    One can see that the new procedure greatly increases the number of frames per second, which is exactly what we wanted.

    Can we have more?

    Yes! we can do better. You may have noticed on the above captures that some parts of the screen seem to be written several times even if the foreground seems opaque. The main reason is that these foreground sprites have some transparent pixels on one of the edges of their box. Thus, the procedure does not consider any opaque box for them. In order to improve this, we just have to compute some kind of opaque box inside each sprite. More precisely, we want the largest opaque box of each sprite. Is it easy to compute? Well, it may be easy, if you reformulate the problem as the largest rectangle with no transparent pixel. You now have an instance of the well known Largest Empty Rectangle problem for which you will find good resources, such as an article named Computing the Largest Empty Rectangle on One- and Two-Dimensional Processor Arrays by Frank Dehne. Contrary to the previous procedure this one cannot be executed at run time (unless you accept the levels to be loaded in several minutes). For our games, we managed to insert the procedure in the level editor, as an optimization step executed when the level is compiled. Then the game engine just has to read the computed opaque boxes and to apply them in the initial procedure.


    Optimizing the performance of the code should not be the main work of the developer but, hopefully, it has to be done sometime. Then it is important to use the best resources available. The measures presented in this articles are done with a simple improvement of the basic procedure which brings great results, then we have found good articles to improve this procedure even more. One can also think of computing k-largest non transparent rectangles in order to obtain more opaque boxes to filter the sprites, in which case comes the question of the limits to apply to the recursion. One important part not explained in this article is how to compute the sub sprites that must be rendered. The procedure is not really difficult but one has to be careful for the special cases like a rotated or mirrored sprite.

    Article Update Log

    10 June 2013: Initial release

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    It seems like you could do this with a depth pre-pass too. Render all your sprites (using the largest opaque box) just to the depth buffer, then do the final pass with the real sprites. Have you tried that? I don't know if it would produce as good performance improvement but it would be simpler on the CPU side.

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    What sort of rendering pipeline are you actually using?  Is this using older-style blitting?  If so, then I think your approach is ideal, although the blitting methods themselves are not very hardware accelerated and thus possibly not the best if you're going to have lots of transparency and other elements.


    You might consider doing some more specific profiling to see exactly what is causing your FPS drops.  What this article concerns is pixel fill rate on the GPU, and certainly that is something that is a limiting factor.  But it's really mainly a limiter on older GPUs at the scale of the graphics you're using here, whereas there are some much tigher bottlenecks even on very new GPUs.


    The constraints I'm speaking of are material swaps (where textures and such are sent repeatedly to the GPU back and forth during one frame) and the number of "draw calls" in many engines, which mostly just means how many vertex sets are sent.


    For myself and my own work, I've focused on reducing material swaps in the main, and then to a lesser extent focused on reducing pixel fill rate and draw calls.


    The main thing with reducing material swaps is sorting the draws per layer in the scene, by material, and then not switching materials between draw calls if the new material is the same as the last.


    For reducing draw calls, my main approach has been to omit compeltely obscured sprites, as you have here.  But also to combine smaller sprites that share a texture into one larger sprite if they are contiguous, which seems the opposite of what you are doing here.  It seems like your approach will lead to a lower pixel fill rate requirement, in other words, but to a larger number of draw calls.  I am not sure this is a good tradeoff, but you'd have to profile it in your specific engine in order to really see.


    There are some other indies I know with vastly more sophisticated techniques than what I'm using.  They are using geometry combination methods that reduce the draw calls even for non-contiguous usages of a given texture, and I believe even across draw depths.  And through the use of sprite dictionaries for way more things than I tend to use them for, they're also achiveing vastly fewer texture swaps, too.


    I'm really impressed with what they are doing, but like you I think that optimization should only be taken so far, since there are always downsides.  In my case I feel like those sprite dictionaries and so forth slow development to an unhelpful degree when there are lots of sprites (one of my games has north of 5000 individual sprite frames), so there's always a tradeoff there.


    Doing the sub bounding boxes that you were suggesting as the next step would certainly be possible, but I think you will get diminishing returns from that compared to what you have already done.  And, as I think you somewhat implied, I think the work for that next step of the sub bounding boxes will be a lot more than the work that it took to get you to this point.


    Anyway -- very good article, and I like the way you were visualizing the pixel fill repetition.  Very clever!  I would suggest texture sorting for material swap reduction, and then geometry combination for draw call reduction, as the next avenues for getting your fps higher, though.



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    At what point does the setup cost of multiple triangles outweigh the cost of overdraw?

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    @phil_t Before implementing this solution I did implement a solution based on the depth test but it was not convincing. It saved the computation of the fragments' color but it still cost the depth test. Nevertheless I have never tried a two passes rendering as you describe.


    @x4000 It is indeed an old style blitting procedure. At the time of the article there was still the possibility of changing the back end, thus I wanted a rendering solution that would still work with another tool. Now the rendering is done using OpenGL and a better ordering of the materials should indeed improve the procedure. Right now there is no optimization of the textures' content nor do we minimize the texture switches. So thanks for the hints :)


    @cdoty I have never seen worst result with this procedure than with the previous one even if, as you have noticed, the new procedure increases the number of vertices. My first reflex was to limit the size of the sub textures but actually it made no difference. Maybe a combination with the improvement proposed by x4000 would lead to different results on this subject.

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    cdoty - it seems unlikely that the author is using hardware acceleration at all (at least directly, if he were then a depth pre-pass would be better, as others have said) so triangle setup is irrelevant to him -- but I can make an educate guess that the answer to your question is "When they are very, very small". Probably something on the order of 16 pixels or even smaller. The same is probably true of the logic that the author implements for his rectangles.


    In what experience I have with similar optimizations, I have always found that the tipping point is very low on the scale.

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    @j-jorge: Ah, ok!  If it's a blitting infrastructure, then absolutely your way (and your proposed second step for improvements) are clearly the best method by far.  Overdraw is such a huge issue with blitting methods, for sure.


    Anyhow, glad to hear the hints might be helpful for your new OpenGL methods.  I too started with blitting methods (DirectX7 back in the 2002-2003 era), and moving to the 3D approach was really worthwhile but also challenging at first.  There are still some vertex optimizations that I'd like to do but don't know how to do in an efficient way.  I need to sit down with it more at some point, but I'd been focused on the low-hanging fruit of the textures and such prior to that.

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    I have made the same approach recentlry and found that this technique can be good only on old video cards or perhaps java or html5 (not tested). On directx and mediocre video card that would be even slower. For example in my case dividing sprite to peaces algorithm tooks me about 0.250 ms for whole scene and there was only 0.050 ms improvement comparing without algorithm... so 0.200 ms worse... and that qould be great for just some parts of scene (for ex background), but not for whole scene (background units, bullets, interier).

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    Very nice article, and thanks for linking to the paper on Largest Empty Rectangle.  I am wondering if you can also provide some information on how you subdivide("shatter") the shapes?  Do you simply divide the rects in 1/2 and continue to divide each subsection that is overlapped in half until the smaller rectangles are not hidden(Similar to a quadtree)?

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    @MikeS I process the shapes from the front to the back while keeping a list L of rectangles representing the parts of the screen not yet covered by an opaque shape. When a new shape is considered, I search the rectangles in L intersecting the shape then, for each of them:


    - I create for a new shape with the part contained in the rectangle,

    - I remove the rectangle from L, subtract the opaque rectangle of the shape to obtain zero to four new rectangles representing the non covered parts, and I insert these new rectangles in L.


    There is no quadtree but… hem… it seems perfect for L. I don't know why I did not used them, I should have thought about it :/ Also, the way I split the non covered parts, as shown by the fifth picture, has a tendency to produce vertical stripes that lead to visual artifacts. I should be easy to produce more regular rectangles.

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