I am making a program that simulates some physical interaction between particles and draws their trails. My current solution is using NSView descendant that has override DrawRect, but it seems like not the best solution for some reason, one being not very high FPS when drawing many objects. Is there any better way like OpenGL, or is it "as good as it gets", and performance boost is little to none?
If UI content changes all time, then OpenGL is definitely correct approach. The idea behind it is that screen refreshes at constant frame rate (ex. 60FPS) and you can draw different picture each frame. Traditional Cocoa views are designed to keep static picture.
There are many frameworks that give you high-level API for drawing, so you deal with view-like sprites and textures, and you are free from OpenGL-specific functions and states. I have experience with Cocos2d and would recommend it.
To clarify the purpose of this question: I know HOW to create complicated views with both subviews and using drawRect. I'm trying to fully understand the when's and why's to use one over the other.
I also understand that it doesn't make sense to optimize that much ahead of time, and do something the more difficult way before doing any profiling. Consider that I'm comfortable with both methods, and now really want a deeper understanding.
A lot of my confusion comes from learning how to make table view scroll performance really smooth and fast. Of course the original source of this method is from the author behind twitter for iPhone (formerly tweetie). Basically it says that to make table scrolling buttery smooth, the secret is to NOT use subviews, but instead do all the drawing in one custom uiview. Essentially it seems that using lots of subviews slows rendering down because they have lots of overhead, and are constantly re-composited over their parent views.
To be fair, this was written when the 3GS was pretty brand spankin new, and iDevices have gotten much faster since then. Still this method is regularly suggested on the interwebs and elsewhere for high performance tables. In fact it's a suggested method in Apple's Table Sample Code, has been suggested in several WWDC videos (Practical Drawing for iOS Developers), and many iOS programming books.
There are even awesome looking tools to design graphics and generate Core Graphics code for them.
So at first I'm lead to believe "there’s a reason why Core Graphics exists. It’s FAST!"
But as soon as I think I get the idea "Favor Core Graphics when possible", I start seeing that drawRect is often responsible for poor responsiveness in an app, is extremely expensive memory wise, and really taxes the CPU. Basically, that I should "Avoid overriding drawRect" (WWDC 2012 iOS App Performance: Graphics and Animations)
So I guess, like everything, it's complicated. Maybe you can help myself and others understand the When's and Why's for using drawRect?
I see a couple obvious situations to use Core Graphics:
You have dynamic data (Apple's Stock Chart example)
You have a flexible UI element that can't be executed with a simple resizable image
You are creating a dynamic graphic, that once rendered is used in multiple places
I see situations to avoid Core Graphics:
Properties of your view need to be animated separately
You have a relatively small view hierarchy, so any perceived extra effort using CG isn't worth the gain
You want to update pieces of the view without redrawing the whole thing
The layout of your subviews needs to update when the parent view size changes
So bestow your knowledge. In what situations do you reach for drawRect/Core Graphics (that could also be accomplished with subviews)? What factors lead you to that decision? How/Why is drawing in one custom view recommended for buttery smooth table cell scrolling, yet Apple advises drawRect against for performance reasons in general? What about simple background images (when do you create them with CG vs using a resizable png image)?
A deep understanding of this subject may not be needed to make worthwhile apps, but I don't love choosing between techniques without being able to explain why. My brain gets mad at me.
Question Update
Thanks for the information everyone. Some clarifying questions here:
If you are drawing something with core graphics, but can accomplish the same thing with UIImageViews and a pre-rendered png, should you always go that route?
A similar question: Especially with badass tools like this, when should you consider drawing interface elements in core graphics? (Probably when the display of your element is variable. e.g. a button with 20 different color variations. Any other cases?)
Given my understanding in my answer below, could the same performance gains for a table cell possibly be gained by effectively capturing a snapshot bitmap of your cell after your complex UIView render's itself, and displaying that while scrolling and hiding your complex view? Obviously some pieces would have to be worked out. Just an interesting thought I had.
Stick to UIKit and subviews whenever you can. You can be more productive, and take advantage of all the OO mechanisms that should things easier to maintain. Use Core Graphics when you can't get the performance you need out of UIKit, or you know trying to hack together drawing effects in UIKit would be more complicated.
The general workflow should be to build the tableviews with subviews. Use Instruments to measure the frame rate on the oldest hardware your app will support. If you can't get 60fps, drop down to CoreGraphics. When you've done this for a while, you get a sense for when UIKit is probably a waste of time.
So, why is Core Graphics fast?
CoreGraphics isn't really fast. If it's being used all the time, you're probably going slow. It's a rich drawing API, which requires its work be done on the CPU, as opposed to a lot of UIKit work that is offloaded to the GPU. If you had to animate a ball moving across the screen, it would be a terrible idea to call setNeedsDisplay on a view 60 times per second. So, if you have sub-components of your view that need to be individually animated, each component should be a separate layer.
The other problem is that when you don't do custom drawing with drawRect, UIKit can optimize stock views so drawRect is a no-op, or it can take shortcuts with compositing. When you override drawRect, UIKit has to take the slow path because it has no idea what you're doing.
These two problems can be outweighed by benefits in the case of table view cells. After drawRect is called when a view first appears on screen, the contents are cached, and the scrolling is a simple translation performed by the GPU. Because you're dealing with a single view, rather than a complex hierarchy, UIKit's drawRect optimizations become less important. So the bottleneck becomes how much you can optimize your Core Graphics drawing.
Whenever you can, use UIKit. Do the simplest implementation that works. Profile. When there's an incentive, optimize.
The difference is that UIView and CALayer essentially deal in fixed images. These images are uploaded to the graphics card (if you know OpenGL, think of an image as a texture, and a UIView/CALayer as a polygon showing such a texture). Once an image is on the GPU, it can be drawn very quickly, and even several times, and (with a slight performance penalty) even with varying levels of alpha transparency on top of other images.
CoreGraphics/Quartz is an API for generating images. It takes a pixel buffer (again, think OpenGL texture) and changes individual pixels inside it. This all happens in RAM and on the CPU, and only once Quartz is done, does the image get "flushed" back to the GPU. This round-trip of getting an image from the GPU, changing it, then uploading the whole image (or at least a comparatively large chunk of it) back to the GPU is rather slow. Also, the actual drawing that Quartz does, while really fast for what you are doing, is way slower than what the GPU does.
That's obvious, considering the GPU is mostly moving around unchanged pixels in big chunks. Quartz does random-access of pixels and shares the CPU with networking, audio etc. Also, if you have several elements that you draw using Quartz at the same time, you have to re-draw all of them when one changes, then upload the whole chunk, while if you change one image and then let UIViews or CALayers paste it onto your other images, you can get away with uploading much smaller amounts of data to the GPU.
When you don't implement -drawRect:, most views can just be optimized away. They don't contain any pixels, so can't draw anything. Other views, like UIImageView, only draw a UIImage (which, again, is essentially a reference to a texture, which has probably already been loaded onto the GPU). So if you draw the same UIImage 5 times using a UIImageView, it is only uploaded to the GPU once, and then drawn to the display in 5 different locations, saving us time and CPU.
When you implement -drawRect:, this causes a new image to be created. You then draw into that on the CPU using Quartz. If you draw a UIImage in your drawRect, it likely downloads the image from the GPU, copies it into the image you're drawing to, and once you're done, uploads this second copy of the image back to the graphics card. So you're using twice the GPU memory on the device.
So the fastest way to draw is usually to keep static content separated from changing content (in separate UIViews/UIView subclasses/CALayers). Load static content as a UIImage and draw it using a UIImageView and put content generated dynamically at runtime in a drawRect. If you have content that gets drawn repeatedly, but by itself doesn't change (I.e. 3 icons that get shown in the same slot to indicate some status) use UIImageView as well.
One caveat: There is such a thing as having too many UIViews. Particularly transparent areas take a bigger toll on the GPU to draw, because they need to be mixed with other pixels behind them when displayed. This is why you can mark a UIView as "opaque", to indicate to the GPU that it can just obliterate everything behind that image.
If you have content that is generated dynamically at runtime but stays the same for the duration of the application's lifetime (e.g. a label containing the user name) it may actually make sense to just draw the whole thing once using Quartz, with the text, the button border etc., as part of the background. But that's usually an optimization that's not needed unless the Instruments app tells you differently.
I'm going to try and keep a summary of what I'm extrapolating from other's answers here, and ask clarifying questions in an update to the original question. But I encourage others to keep answers coming and vote up those who have provided good information.
General Approach
It's quite clear that the general approach, as Ben Sandofsky mentioned in his answer, should be "Whenever you can, use UIKit. Do the simplest implementation that works. Profile. When there's an incentive, optimize."
The Why
There are two main possible bottlenecks in an iDevice, the CPU and GPU
CPU is responsible for the initial drawing/rendering of a view
GPU is responsible for a majority of animation (Core Animation), layer effects, compositing, etc.
UIView has a lot of optimizations, caching, etc, built in for handling complex view hierarchies
When overriding drawRect you miss out on a lot of the benefits UIView's provide, and it's generally slower than letting UIView handle the rendering.
Drawing cells contents in one flat UIView can greatly improve your FPS on scrolling tables.
Like I said above, CPU and GPU are two possible bottlenecks. Since they generally handle different things, you have to pay attention to which bottleneck you are running up against. In the case of scrolling tables, it's not that Core Graphics is drawing faster, and that's why it can greatly improve your FPS.
In fact, Core Graphics may very well be slower than a nested UIView hierarchy for the initial render. However, it seems the typical reason for choppy scrolling is you are bottlenecking the GPU, so you need to address that.
Why overriding drawRect (using core graphics) can help table scrolling:
From what I understand, the GPU is not responsible for the initial rendering of the views, but is instead handed textures, or bitmaps, sometimes with some layer properties, after they have been rendered. It is then responsible for compositing the bitmaps, rendering all those layer affects, and the majority of animation (Core Animation).
In the case of table view cells, the GPU can be bottlenecked with complex view hierarchies, because instead of animating one bitmap, it is animating the parent view, and doing subview layout calculations, rendering layer effects, and compositing all the subviews. So instead of animating one bitmap, it is responsible for the relationship of bunch of bitmaps, and how they interact, for the same pixel area.
So in summary, the reason drawing your cell in one view with core graphics can speed up your table scrolling is NOT because it's drawing faster, but because it is reducing the load on the GPU, which is the bottleneck giving you trouble in that particular scenario.
I am a game developer, and I was asking the same questions when my friend told me that my UIImageView based view hierarchy was going to slow down my game and make it terrible. I then proceeded to research everything I could find about whether to use UIViews, CoreGraphics, OpenGL or something 3rd party like Cocos2D. The consistent answer I got from friends, teachers, and Apple engineers at WWDC was that there won't be much of a difference in the end because at some level they are all doing the same thing. Higher-level options like UIViews rely on the lower level options like CoreGraphics and OpenGL, just they are wrapped in code to make it easier for you to use.
Don't use CoreGraphics if you are just going to end up re-writing the UIView. However, you can gain some speed from using CoreGraphics, as long as you do all your drawing in one view, but is it really worth it? The answer I have found is usually no. When I first started my game, I was working with the iPhone 3G. As my game grew in complexity, I began to see some lag, but with the newer devices it was completely unnoticeable. Now I have plenty of action going on, and the only lag seems to be a drop in 1-3 fps when playing in the most complex level on an iPhone 4.
Still I decided to use Instruments to find the functions that were taking up the most time. I found that the problems were not related to my use of UIViews. Instead, it was repeatedly calling CGRectMake for certain collision sensing calculations and loading image and audio files separately for certain classes that use the same images, rather than having them draw from one central storage class.
So in the end, you might be able to achieve a slight gain from using CoreGraphics, but usually it will not be worth it or may not have any effect at all. The only time I use CoreGraphics is when drawing geometric shapes rather than text and images.
I need to draw onto a very, very wide context (possibly going into the 100,000's of pixels in width)
The stuff that I am drawing is simple (exclusively lines, ellipses and arcs).
Currently, I am using a tiling mechanism to render pieces of this image as SVG graphics in UIWebViews sitting in a UIScrollView. Horrible, I know. Performance is so bad it makes me want to hurt myself.
So I'm left with no choice but to draw things directly in Quartz. The problem is that I couldn't find any good information on handling such a large drawing.
The commands will be stored in a array (i.e [0] - line from x1,x2 to y1,2 [2] - arc at x1,x2,h,v etc...) which makes it possible to immediately determine what needs to be drawn at any point of time.
How should I best go about handling the actual drawing? Make a ginormous context then draw onto it, erasing the parts that go off screen? Can I draw straight into the UIScrollView?
Or am I supposed to performing some sort of tiling? Creating smaller views and drawing on them?
Look into CATiledLayer. It supports huge dimensions, yet only draws "tiles" that are visible. This means that in your drawLayer:inContext: you need to calculate how to draw the tile you were asked to draw. That might be quite complicated to implement but the performance boost makes up for this. I use this to draw a "strip" that is often over 20.000 pixels wide, tiled into 256 pixel pieces (this is the default).
You not only want this because of speed but also due to memory: if you really had a view or layer that were that big it would take up immense amounts of memory. Tiling is the only solution here.
See this nice article by Michael Heyeck on how to use CATiledLayer (Apple has a demo as well, but I think the linked article is better).
If the stuff you are drawing is not static then using CATiledLayer(s) doesn't work very well. A better solution is presented partly in http://www.mlsite.net/blog/?p=1342 . I've used this as a basis from some interactive drawing. A little slow but I'm still optimizing. Use http://red-glasses.com/index.php/tutorials/catiledlayer-how-to-use-it-how-it-works-what-it-does/ as a check list of whether you should use CATiledLayer(s) or not.
When is it recommended to use gradient created by the iphone itself and when is an image the better choice?
This question asks something similar. As with most performance-related questions, you are best served by using Instruments and other tools to determine if this is an area worth spending the time to optimize.
As I state in my answer, I had noticed a significant amount of time being spent during launch in the Quartz functions for drawing a radial gradient in the background on an iPhone 3G. By switching to an image, I was able to noticeably reduce startup time of my application. However, a new image would need to be generated for each larger display size, so for the newer devices (iPad, iPhone 4), I use the Quartz radial gradient once again because of the negligible rendering time on those systems.
For linear gradients, it has been my experience that a CAGradientLayer gives you both good performance and scalability to new resolutions, but again you'll need to test this in your particular application.
That is a pretty vague question, are you thinking about any particular situations? Unless you are doing very intensive processing or heavy graphics (game) then the difference in processing time is probably not going to be noticeable. I haven't measured it, but I would bet the difference in overhead is pretty minimal, so do whichever you think is easier/nicer looking until you find a problem.
I'm writing a game that displays 56 hexagon pieces filling the screen in the shape of a board. I'm currently drawing each piece using a singleton rendering class that when called to draw a piece, creates a path from 6 points based of the coordinate passed in. This path is filled with a solid color and then a 59x59 png with an alpha to white gradient is overlayed over the drawing to give the piece a shiny look. Note I'm currently doing this in Core Graphics.
My first thought is that creating a path everytime I draw is costly and seems like I can somehow do this once and then reuse it, but I'm not sure of the best approach for this. When I look at the bottlenecks with Shark, it looks like the drawing of the png is the most taxing part of the process. I've tried just rendering the png overlay or just rendering the path without the overlay and both give me some frame gains, although removing the png overlay yields the most frames.
My current thought is that at startup, I should render 6 paths (1 for each color piece I have) and overlay them with the png and then store an image of these pieces and then just redraw the pieces each time I need them. Is there an effecient machanism for storing something you've drawn once and redrawing it? It kinda just sounds like I'd be running into the whole drawing pngs too often thing again, but maybe there's a less taxing method that does a similar thing...
Any suggestions are much appreciated.
Thanks!
You might try CGLayer or CALayer.
General thoughts:
Game programming on iPhone usually necessitates OpenGL. Core Graphics is a bit easier to work with, but OpenGL is optimized for speed.
Prerender this "shiny look" into the textures as much as is possible (as in: do it in Photoshop before you even insert them into your project). Alpha blending is hell on performance.
Maybe try PVRTC (also this tutorial) as it's a format used by iPhone's GPU's manufacturer. Then again, this could make things worse depending on where your bottleneck is.
If you really need speed you have to go the OpenGL route. Be careful if you want to mix OpenGL and Core Animation, they can conflict.
OpenGL is a pain if you haven't done much with it. It sounds like you could use Core Animation and make each tile a layer. CA doesn't call the redraw again unless you change something, so you should be able to just move that layer around without taking a big hit. Also note that CA stores the layer in the texture memory so it should be much faster.
Some others have mentioned that you should use OpenGL. Here's a nice introduction specifically for the iPhone: OpenGL ES from the Ground Up: Table of Contents
You might also want to look at cocos2d. It seems to be significantly faster than using CoreAnimation in my tests, and provides lots of useful stuff for games.