Talking in context of a game based on openGL renderer :
Lets assume there are two threads :
that updates the gameLogic and physics etc. for the in game objects
that makes openGL draw calls for each game object based on data in the game objects (that thread 1 keeps updating)
Unless you have two copies of each game object in the current state of the game you'll have to pause Thread 1 while Thread 2 makes the draw calls otherwise the game objects will get updated in the middle of a draw call for that object ! which is undesirable!
but stopping thread 1 to safely make draw calls from thread 2 kills the whole purpose of multithreading/cocurrency
Is there a better approach for this other than using hundreds or thousands or sync objects/fences so that the multicore architecture can be exploited for performance?
I know I can still use multiThreading for loading texture and compiling shaders for the objects which are yet to be the part of the current game state but how do I do it for the active/visible objects without causing conflict with draw and update?
The usual approach is that the simulation thread after completing a game step commits the state into an intermediary buffer and then signals the renderer thread. Since OpenGL executes asynchronously the render thread should complete rather quickly, thereby releasing the intermediary buffer for the next state.
You shouldn't render directly from the game state anyway, since what the renderer needs to do its works and what the simulation produces not always are the same things. So some mapping may be necessary anyway.
This is quite a general question you're asking. If you ask 10 different people, you'll probably get 10 different answers. In the past I implemented something similar, and here's what I did (after a long series of optimisation cycles).
Your model-update loop which runs on a background thread should look something like this:
while(true)
{
updateAllModels()
}
As you said, this will cause an issue when the GL thread kicks in, since it may very well render a view based on a model which is half way through being rendered, which can cause UI glitches at the best case.
The straight-forward way for dealing with this would be synchronising the update:
while (true)
{
synchronized(...)
{
updateAllModels();
}
}
Where the object you synchronize with here is the same object you'll use to synchronize the drawing method.
Now we have an improved method which won't cause glitches in the UI, but the overall rendering will probably take a very severe performance hit, since all rendering needs to wait until all model updates are finished, or vise versa - the models update will need to wait until all drawing is finished.
Now, lets think for a moment - what do we really need to be synchronizing?
In my app (a space game), when updating the models, I needed to calculate vectors, check for collisions and update all the object's positions, rotations, scale, etc.
Out of all these things, the only things the view cares about is the position, rotation, scale and a few other small considerations which the UI needs to know in order to correctly render the game world. The rendering process doesn't care about a game object's vector, the AI code, collision tests, etc. Considering this, I altered my update code to look something like this:
while (true)
{
synchronized(...)
{
updateVisibleChanges(); // sets all visible changes - positions, rotations, etc
}
updateInvisibleChanges(); // alters vectors, AI calculations, collision tests, etc
}
Same as before, we're synchronising the update and the draw methods, but this time, the critical section is much smaller than before. Essentially, the only things which should be set in the updateVisibleChanges method are things which pertain to the position, rotation, scale, etc of the objects which should be rendered. All other calculations (which are usually the most exhaustive ones) are performed afterwards, and do not stop the rendering from occurring.
An added bonus from this method - when you're performing your invisible changes, you can be sure that all objects are in the position they need to be (which is very useful for accurate collision tests). For example, in the method before the last one, object A moves, then object A tests a collision against object B which hasn't moved yet. It is possible that had object B moved before object A tested a collision, there would be a different result.
Of course, the last example I showed isn't perfect - you will still need to hang the rendering method and/or the updateVisible method to avoid clashes, but I fear that this will always be a problem, and the key is minimizing the amount of work you're doing in either thread sensitive method.
Hope this helps :)
Related
Disclaimer: it's possible I'm going about this in completely the wrong way & trying to solve the wrong problem.
I'm writing an Android app that uses Google Maps to show cloud cover over a 1km square area by adding a 1km-diameter circle to the map for each area with detected cloud cover via the following (probably naive) method:
CircleOptions options = new CircleOptions();
// ... set the center, radius, fillColor, strokeWidth, and strokeColor...
mMap.addCircle(options);
If there are just a few dozen circles added to the map, the performance is fine. If half the state of Florida is covered with clouds, the program chokes & hangs the UI for 5-30 seconds while it makes a few thousand calls to addCircle()
AsyncTask won't work here, because the time-consuming jobs are the calls to mMap.addCircle(), which HAVE to be made from the UI thread. I actually tried using doInBackground() to populate an array of CircleOptions, then iterated through it in onPostExecute() to make the calls to addCircle()... and as expected, it made absolutely no difference whatsoever to the app's performance (I think populating the CircleOptions array in doInBackground() shaved a whopping 18ms from the execution time on the UI thread).
In Unity, I could just use the Update() lifecycle method (which gets called once per video frame) and make 20-50 calls to addCircle() at a time before exiting to give it time to metaphorically go up for air and service the UI before the next batch. But AFAIK, Android Activity has no equivalent to this. You can do things that don't require UI changes in a background thread, then update the UI when it finishes... but there doesn't seem to be any explicit provision in Android for gracefully spreading multiple time-consuming UI updates across multiple frames.
So... DOES Android have some way to gracefully batch time-consuming UI updates across multiple frames a-la-Unity? Or is there a better way to render circle overlays onto a Google Map that can scale gracefully and handle thousands of such circles without choking?
In OpenGL Renderer onDrawFrame is called several time, until the page is completely rendered. I cannot find an event that my page is completeley rendered in order to take a snapshot of the OpenGL page and animate it.
I have the solution to take snapshot on at the animation trigger (specific button), but this will imply a specific delay, until Bitmap is created, such as i would like to keep in memory a mutable copy of every page.
Do you know other way to animate GLSurfaceView with rendered content?
Snippet for triggering snapshot.
glSurfaceView.queueEvent(new Runnable() {
#Override
public void run() {
glSurfaceView.getRenderer().takeGlSnapshot();
}
});
EGLContext for passing the GL11 object.
public void takeGlSnapshot() {
EGL10 egl = (EGL10) EGLContext.getEGL();
GL11 gl = (GL11) egl.eglGetCurrentContext().getGL();
takeSnapshot(gl);
}
onDrawFrame(Gl10 gl) {
//is last call for this page event ????????????
No such event exists, as I will explain below.
OpenGL is designed as a client/server architecture, with the two running asynchronously. In a modern implementation, you can generally think of the client as the API front-end that you use to issue commands and the server as the GPU/driver back-end. API calls will do a little bit of work to validate input parameters etc, but save for a few exceptions (like glReadPixels (...)) they buffer up a command for the server to execute at a later point. You never truly know when your commands are finished, unless you explicitly call glFinish (...).
Calling glFinish (...) at the end of each frame is an awful idea, as it will create a CPU/GPU synchronization point and undo the benefits of having the CPU and GPU run asynchronously. But, if you just want to take a screenshot of the current frame every once in a while, then glFinish (...) could be an acceptable practice.
Another thing to consider, if you are using double-buffered rendering is that you may be able to access the last fully rendered frame by reading the front-buffer. This is implementation specific behavior however, as some systems are designed to discard the contents of the front-buffer after the buffer swap operation, others make reading its contents an undefined operation. In any case, if you do attempt this solution, be aware that the image returned will have a 1 frame latency (however, the process of reading it will not require you to finish your current frame), which may be unacceptable.
Hi Android Developers,
What is the best way to interrupt a current rendering phase of GLSurfaceView and start a new one when mode is equal to "Render_when_dirty"? I artificially stop rendering in "onDraw" method by checking a flag and returning from actual rendering method which is called in "onDraw" method; then, in main thread's context i call "requestRender()" to refresh the scene. However, due to a reason that i am not aware of, some of the intermediary old frames are displayed for a very very short period of time(on the other hand, they endure for so long period of time that users can realize the transition); before actual scene is rendered by opengl es 2.x engine. It doesn't affect anything at all; but troublesome to be fixed. What do you suggest?
P.S. Throwing InterruptedException within onDraw method is useless due to the destruction of actual rendering thread of GLSurfaveView.
Kind Regards.
When you say some of the old frames are drawn - do you mean part of the frame that is drawn is old or multiple calls of onDraw() still lead to some of the old information being shown on the display.
There are a few things I can see happening here. If you have a onDraw() like this:
onDrawFrame(){
... stuff ...
if (stateVariableSet)
return;
... stuff ...
my understanding is that when the function is done being run, that the back/front buffer get swapped and drawn. One thing that could be happening here is that you see a few calls of onDrawFrame() being rendered while you try to update the state/State variable.
On the other hand, if you have something like this:
onDrawFrame(){
... stuff..
semaphore.acquire(); // lock the thread waiting for the state to update
... stuff ...
then the things that have been drawn before the lock will be stale (for that frame only though - at least that's what I'd anticipate).
Also are you running on a multi-core system?
I have got some strange and unexpected results from my program in OpenGL ES for android for example in the code below:
matrix = ThisRot.get();
gl.glMultMatrixf(matrix, 0);
currentRotation.toMatrix(matrix);
temp.set(matrix);
I set the matrix value before I use it as an argument for gl.glMultMatrixf and after that I change the value of matrix and use it for another purpose, but it has effect an the way the object rotate so it should have effect on gl.glMultMatrixf(). and that's not the only one, some other places in my code I had this unexpected results. so I have thought maybe these happen due to mutual exclusion and multitreading and those kind of things.
am I right? should we worry about multithreading when we code in Opengl ES for android? How can I avoid these kind of problems.
Of course you should worry about multithreading. In particular, Android creates its own GLThread for rendering, when you attach a GLRenderer-derived class to a GLSurfaceView using the setRenderer() function.
In fact, multithreading can cause crashes (not only unexpected behavior) in your programs especially when you loop through arrays adding/removing objects and such.
Check if you are modifying the same data inside the onDrawFrame function of your GLRenderer and your own thread. If you are, try adding the following around the modified code (in both threads):
synchronize(variable) {
modify(variable);
}
This will lock the variable throughout the modify() function until it ends the block. Try not to overuse it, though, only in places where you need it. One thread will block the other one until it's finished!
I use opengl to draw values from an array. The values are from the 3 Android sensors. I want to turn of the renderer during I collect the values. It seems that drawing and gathering at the same time have differenet results, so I want to turn off rendering while I gathering sensorvalues. At the moment I use a boolean in the ondraw() to stop drawing. But is there a better way to do this? There happen freaky things when I stop drawing , like the stuff I draw jittering a little bit some times, but it can't be because there are no drawing( ondraw(){ if(boolean){.........}}
Thank you so much for your help :-)
You can ask GLSurfaceView to only draw when asked via setRenderMode(GLSurfaceView.RENDERMODE_WHEN_DIRTY), then call requestRender() when you're ready to draw.
I have a suggestion: Create a thread to collect the sensor data, and make it available as an object. This thread would gather all the data, and synchronize on the public value object. Blocking time would be minimized.
collectLocalData = getData();
synchronized(pubData) {
pubData.set(collectLocalData);
}
Your render thread can then Do the same when copying the public values for local consumption.
synchronized(pubData) {
renderLocalData.set( pubData );
}
renderData( renderLocalData );
If polling the sensors takes longer than rendering time, the same data will be rendered more than once. The animation might get a little twitchy, but no more so than what the data represents.
PS: your jitters may be a result of the data itself rather than your code.