# OpenGL OpenGL and multi-threaded applications...

This topic is 4684 days old which is more than the 365 day threshold we allow for new replies. Please post a new topic.

## Recommended Posts

##### Share on other sites
Quote:

Is there any specific reason that you wish to use threads in a case like the above. Graphics/Rendering dont thread well for various reasons and I would suggest against using them (fine for physics, AI, etc if you need it) but they'll probably slow down your rendering quite a bit.

In any case have a look at sempaphores (can used for synchronization) and maybe the producer/consumer pattern.

HTH

##### Share on other sites

Rendering is in my case only secondary. If it halts from time to time this is not important at all. Threads are critical in my case since there is socket communication, serial communication, etc. and basically the application is a server application that gets commands from another application. Synchronisation between threads is not my problem, I just want to be able to use thread A and thread B in turn for rendering purposes (or rather thread A will always render, but thread B will create new objects to render (like textures)).

##### Share on other sites
OpenGL does support multithreading; however, only one thread can access a context at a time, so each thread will need it's own thread. See MSDN KB Q128122 for an example using wgl (I assume you're using Windows-- if not, it can be done with glX, agl, etc. as well, but obviously the particular methods will be different)

It's worth noting, though, that since the threads use different contexts, a context switch overhead occurs since they are almost definitely using a graphics card. That is, the driver serializes the calls, with added context switch overhead, so don't expect to get blistering performance gains on most architectures (at least not yet). However, for the situation you've described of loader threads, this solution is quite nice. I use it in my engine.

Hope this helps,
-bodisiw

##### Share on other sites
The only hard rule is that only one thread can have a context selected at a time. You can deselect the context, and select it another thread though. If you set up the context to use shared display list you ought to be able to share other resources as well (otherwise how could the shared display list work?).

I started working on this, but never took it to completion. I was focusing on having a seperate rendering thread from the windows message pump at the time.

Switching OGL contextes is very expensive; you have to employ a monitor syncronization scheme so that you only (de)select when you absolutely must.

##### Share on other sites
Thank you for the quick replies.

I tried it but now somehow my main thread can't get the control back...
Here is how I switch between threads:

wglMakeCurrent(NULL,NULL);
sleepUntilWakingUpFlagSet();
wglMakeCurrent(hDC,hRC);

wglMakeCurrent(hDC,hRC);
DoSomeOpenGLStuff();
wglMakeCurrent(NULL,NULL);
setWakingUpFlag()

In both cases hDC and hRC are the same (the ones created at the beginning of the application by the rendering thread).
What am I doing wrong?

##### Share on other sites
If you're not too fussed about performance you could do it as follows using a mutex;
mutextype mutex;// rendering threadmutex.lock();    // lock the mutex or wait until we canwglMakeCurrent(hDC,hRC);  // get the rendering contextrenderstuff();wglMakeCurrent(NULL,NULL); // release rendering contextmutex.release();// worker threadmutex.lock()wglMakeCurrent(hDC,hRC);DoSomeOpenGLStuff();wglMakeCurrent(NULL,NULL);mutex.release();

You can ofcourse put some flags in place to prevent it aquring the context when its not needed.

The other option would be a simple signal system, so your code would be more like;

// rendering threadwhile(!rendertime){ sleep(0);}wglMakeCurrent(hDC,hRC);  // get the rendering contextrenderstuff();wglMakeCurrent(NULL,NULL); // release rendering contextrendertime = false;// worker threadwglMakeCurrent(hDC,hRC);DoSomeOpenGLStuff();wglMakeCurrent(NULL,NULL);rendertime = true;while(rendertime){ sleep(0);}

very basic syncing system, but it should do the trick.
Just make sure before you kick your worker off that rendertime = false and that the main thread has given up the context.

You could go more over the top with slots and signals , but thats just over kill [wink]

##### Share on other sites
Thanks a lot for all replies, it works nicely now :)

• 9
• 10
• 12
• 10
• 10
• ### Similar Content

• Good Evening,
I want to make a 2D game which involves displaying some debug information. Especially for collision, enemy sights and so on ...
First of I was thinking about all those shapes which I need will need for debugging purposes: circles, rectangles, lines, polygons.
I am really stucked right now because of the fundamental question:
Where do I store my vertices positions for each line (object)? Currently I am not using a model matrix because I am using orthographic projection and set the final position within the VBO. That means that if I add a new line I would have to expand the "points" array and re-upload (recall glBufferData) it every time. The other method would be to use a model matrix and a fixed vbo for a line but it would be also messy to exactly create a line from (0,0) to (100,20) calculating the rotation and scale to make it fit.
If I proceed with option 1 "updating the array each frame" I was thinking of having 4 draw calls every frame for the lines vao, polygons vao and so on.
In addition to that I am planning to use some sort of ECS based architecture. So the other question would be:
Should I treat those debug objects as entities/components?
For me it would make sense to treat them as entities but that's creates a new issue with the previous array approach because it would have for example a transform and render component. A special render component for debug objects (no texture etc) ... For me the transform component is also just a matrix but how would I then define a line?
Treating them as components would'nt be a good idea in my eyes because then I would always need an entity. Well entity is just an id !? So maybe its a component?
Regards,
LifeArtist
• By QQemka
Hello. I am coding a small thingy in my spare time. All i want to achieve is to load a heightmap (as the lowest possible walking terrain), some static meshes (elements of the environment) and a dynamic character (meaning i can move, collide with heightmap/static meshes and hold a varying item in a hand ). Got a bunch of questions, or rather problems i can't find solution to myself. Nearly all are deal with graphics/gpu, not the coding part. My c++ is on high enough level.
Let's go:
Heightmap - i obviously want it to be textured, size is hardcoded to 256x256 squares. I can't have one huge texture stretched over entire terrain cause every pixel would be enormous. Thats why i decided to use 2 specified textures. First will be a tileset consisting of 16 square tiles (u v range from 0 to 0.25 for first tile and so on) and second a 256x256 buffer with 0-15 value representing index of the tile from tileset for every heigtmap square. Problem is, how do i blend the edges nicely and make some computationally cheap changes so its not obvious there are only 16 tiles? Is it possible to generate such terrain with some existing program?
Collisions - i want to use bounding sphere and aabb. But should i store them for a model or entity instance? Meaning i have 20 same trees spawned using the same tree model, but every entity got its own transformation (position, scale etc). Storing collision component per instance grats faster access + is precalculated and transformed (takes additional memory, but who cares?), so i stick with this, right? What should i do if object is dynamically rotated? The aabb is no longer aligned and calculating per vertex min/max everytime object rotates/scales is pretty expensive, right?
Drawing aabb - problem similar to above (storing aabb data per instance or model). This time in my opinion per model is enough since every instance also does not have own vertex buffer but uses the shared one (so 20 trees share reference to one tree model). So rendering aabb is about taking the model's aabb, transforming with instance matrix and voila. What about aabb vertex buffer (this is more of a cosmetic question, just curious, bumped onto it in time of writing this). Is it better to make it as 8 points and index buffer (12 lines), or only 2 vertices with min/max x/y/z and having the shaders dynamically generate 6 other vertices and draw the box? Or maybe there should be just ONE 1x1x1 cube box template moved/scaled per entity?
What if one model got a diffuse texture and a normal map, and other has only diffuse? Should i pass some bool flag to shader with that info, or just assume that my game supports only diffuse maps without fancy stuff?
There were several more but i forgot/solved them at time of writing
• By RenanRR
Hi All,
I'm reading the tutorials from learnOpengl site (nice site) and I'm having a question on the camera (https://learnopengl.com/Getting-started/Camera).
I always saw the camera being manipulated with the lookat, but in tutorial I saw the camera being changed through the MVP arrays, which do not seem to be camera, but rather the scene that changes:
#version 330 core layout (location = 0) in vec3 aPos; layout (location = 1) in vec2 aTexCoord; out vec2 TexCoord; uniform mat4 model; uniform mat4 view; uniform mat4 projection; void main() { gl_Position = projection * view * model * vec4(aPos, 1.0f); TexCoord = vec2(aTexCoord.x, aTexCoord.y); } then, the matrix manipulated:
..... glm::mat4 projection = glm::perspective(glm::radians(fov), (float)SCR_WIDTH / (float)SCR_HEIGHT, 0.1f, 100.0f); ourShader.setMat4("projection", projection); .... glm::mat4 view = glm::lookAt(cameraPos, cameraPos + cameraFront, cameraUp); ourShader.setMat4("view", view); .... model = glm::rotate(model, glm::radians(angle), glm::vec3(1.0f, 0.3f, 0.5f)); ourShader.setMat4("model", model);
So, some doubts:
- Why use it like that?
- Is it okay to manipulate the camera that way?
-in this way, are not the vertex's positions that changes instead of the camera?
- I need to pass MVP to all shaders of object in my scenes ?

What it seems, is that the camera stands still and the scenery that changes...
it's right?

Thank you

• Sampling a floating point texture where the alpha channel holds 4-bytes of packed data into the float. I don't know how to cast the raw memory to treat it as an integer so I can perform bit-shifting operations.

int rgbValue = int(textureSample.w);//4 bytes of data packed as color
// algorithm might not be correct and endianness might need switching.
vec3 extractedData = vec3(  rgbValue & 0xFF000000,  (rgbValue << 8) & 0xFF000000, (rgbValue << 16) & 0xFF000000);
extractedData /= 255.0f;

• While writing a simple renderer using OpenGL, I faced an issue with the glGetUniformLocation function. For some reason, the location is coming to be -1.
Anyone has any idea .. what should I do?