I have an OpenGL 2.1 app wherein the user can load multiple images, which can vary in width and height, as textures in order to be rendered on their respective quad objects on the screen.
I was initially pre-loading all the images right away prior to rendering them, but the issue occurs when the user decides to load by the hundreds, on rare cases by the thousands, which results a very high usage of RAM. I am also de-allocating the bitmap container immediately upon converting them to textures, FYI. In addition, I tried reducing the size of the images based on the sizes of their respective quads to be rendered to. But that seems to be only applying a bandage to the issue, as there's the possibility the user can load more images and load more quad objects; hence more RAM.
Sans putting a limit on how much images they can upload, I'm at a loss on how to properly manage textures. I have also read a technique on using Pixel Buffer Objects: transfer image data to the buffer -> render on one re-usable texture, repeat process. But I'm a bit stumped on how to proceed from there, seeing as there seems to be an assumption that all the images have to be the same size prior to updating the texture. There's also the possibility of performance loss, such as drastic decrease in frame rates during the process of uploading images as textures to OpenGL. Though, I'm very much willing to be proven wrong with this.
Can anyone shed some light on this issue or point me in the right direction?
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Currently, My app is using a large amount of memory after loading textures (~200Mb)
I am loading the textures into a char buffer, passing it along to OpenGL and then killing the buffer.
It would seem that this memory is used by OpenGL, which is doing its own texture management internally.
What measures could I take to reduce this?
Is it possible to prevent OpenGL from managing textures internally?
One typical solution is to keep track of which textures you are needing at a given position of your camera or time-frame, and only load those when you need (opposed to load every single texture at the loading the app). You will have to have a "manager" which controls the loading-unloading and bounding of the respective texture number (e.g. a container which associates a string, name of the texture, with an integer) assigned by the glBindTexture)
Other option is to reduce the overall quality/size of the textures you are using.
It would seem that this memory is used by OpenGL,
Yes
which is doing its own texture management internally.
No, not texture management. It just need to keep the data somewhere. On modern systems the GPU is shared by several processes running simultanously. And not all of the data may fit into fast GPU memory. So the OpenGL implementation must be able to swap data out. The GPU fast memory is not storage, it's just another cache level. Just like the system memory is cache for system storage.
Also GPUs may crash and modern drivers reset them in situ, without the user noticing. For this they need a full copy of the data as well.
Is it possible to prevent OpenGL from managing textures internally?
No, because this would either be tedious to do, or break things. But what you can do, is loading only the textures you really need for drawing a given scene.
If you look through my writings about OpenGL, you'll notice that for years I tell people not to writing silly things like "initGL" functions. Put everything into your drawing code. You'll go through a drawing scheduling phase anyway (you must sort translucent objects far-to-near, frustum culling, etc.). That gives you the opportunity to check which textures you need, and to load them. You can even go as far and load only lower resolution mipmap levels so that when a scene is initially shown it has low detail, and load the higher resolution mipmaps in the background; this of course requires appropriate setting of minimum and maximum mip levels to be set as either texture or sampler parameter.
I'm writing a 2D platformer game using SDL with C++. However I have encountered a huge issue involving scaling to resolution. I want the the game to look nice in full HD so all the images for the game have been created so that the natural resolution of the game is 1920x1080. However I want the game to scale down to the correct resolution if someone is using a smaller resolution, or to scale larger if someone is using a larger resolution.
The problem is I haven't been able to find an efficient way to do this.I started by using the SDL_gfx library to pre-scale all images but this doesn't work as it creates a lot of off-by-one errors, where one pixel was being lost. And since my animations are contained in one image when the animation would play the animation would slightly move up or down each frame.
Then after some looking round I have tried using opengl to handle the scaling. Currently my program draws all the images to a SDL_Surface that is 1920x1080. It then converts this surface to a opengl texture, scales this texture to the screen resolution, then draws the texture. This works fine visually but the problem is that its not efficient at all. Currently I am getting a max fps of 18 :(
So my question is does anyone know of an efficient way to scale the SDL display to the screen resolution?
It's inefficient because OpenGL was not designed to work that way. Main performance problems with current design:
First problem: You're software rasterizing with SDL. Sorry, but no matter what you do with this configuration, that will be a bottleneck. At a resolution of 1920x1080, you have 2,073,600 pixels to color. Assuming it takes you 10 clock cycles to shade each 4-channel pixel, on a 2GHz processor you're running a maximum of 96.4 fps. That doesn't sound bad, except you probably can't shade pixels that fast, and you still haven't done AI, user input, game mechanics, sound, physics, and everything else, and you're probably drawing over some pixels at least once anyway. SDL_gfx may be quick, but for large resolutions, the CPU is just fundamentally overtasked.
Second problem: Each frame, you're copying data across the graphics bus to the GPU. This is the slowest thing you can possibly do graphics-wise. Image data is probably the worst of that, because there's typically so much of it. Basically, each frame you're telling the GPU to copy two million some pixels from RAM to VRAM. According to Wikipedia, you can expect, for 2,073,600 pixels at 4 bytes each, no more than 258.9 fps, which again doesn't sound bad until you remember everything else you need to do.
My recommendation: switch your application completely to OpenGL. This removes the need to render to a texture and copy to the screen--just render directly to the screen! Also, scaling is handled automatically by your view matrix (glOrtho/gluOrtho2D for 2D), so you don't have to care about the scaling issue at all--your viewport will just show everything at the same scale. This is the ideal solution to your problem.
Now, it comes with the one major drawback that you have to recode everything with OpenGL draw commands (which is work, but not too hard, especially in the long run). Short of that, you can try the following ideas to improve speed:
PBOs. Pixel buffer objects can be used to address problem two by making texture loading/copying asynchronous.
Multithread your rendering. Most CPUs have at least two cores and on newer chips two register states can be saved for a single core (Hyperthreading). You're essentially duplicating how the GPU solves the rendering problem (have a lot of threads going). I'm not sure how thread safe SDL_gfx is, but I bet that something could be worked out, especially if you're only working on different parts of the image at the same time.
Make sure you pay attention to what place your draw surface is in SDL. It should probably be SDL_SWSURFACE (because you're drawing on the CPU).
Remove VSync. This can improve performance, even if you're not running at 60Hz
Make sure you're drawing your original texture--DO NOT scale it up or down to a new one. Draw it at a different size, and let the rasterizer do the work!
Sporadically update: Only update half the image at a time. This will probably close to double your "framerate", and it's (usually) not noticeable.
Similarly, only update the changing parts of the image.
Hope this helps.
I understand that you usually create complex 3D models in Blender or some other 3D modelling software and export it afterwords as .obj. This .obj file gets parsed into your program and openGL will render it. This as far as I understand real-time rendering.
Now I was wondering if there is something like pre-rendered objects. I'm a little bit confused because there are so many articles/videos about real-time rendering but I haven't found any information about none real-time rendering. Does something like this exists or not? The only thing which would come into my mind as none real-time rendering would be a video.
I guess this is pretty much a yes or no question :) but if it exists maybe someone could point me to some websites with explanations.
"Real-time rendering" means that the frames are being generated as fast as they can be displayed. "Non-real-time rendering", or "offline rendering" means generating frames one at a time, taking as much time as necessary to achieve the desired image quality, and then later assembling them into a movie. Video at the quality of video games can be rendered in real time; something as elaborate as a Pixar movie, though, has to be done in offline mode. Individual frames can still take hours of rendering time!
It's not entirely clear what you mean by "prerendered objects", however there are things called VBOs and Vertex Arrays that store the object's geometry in VRAM so as to not have to load it into the rendering pipeline using glVertex3f() or similar every frame. This is called Immediate Mode.
VBOs and Vertex arrays are used instead of immediate mode because they're far faster than calling the graphics driver to load data into VRAM for every vertex because they are kept in VRAM, which is faster than normal RAM, ready to be booted into the render pipeline.
The page here may help, too.
There's nothing stopping you from rendering to an off-screen frame-buffer (i.e., an FBO) and then saving that to disk rather than displaying it to the screen. For instance, that's how GPGPU techniques used to work before the advent of CUDA, OpenCL, etc. ... You would load your data as an unfiltered floating point texture, perform your calculation using pixel-shaders on the FBO, and then save the results back to disk.
In the link I posted above, it states in the overview:
This extension defines a simple interface for drawing to rendering
destinations other than the buffers provided to the GL by the
window-system.
It then goes on to state,
By allowing the use of a framebuffer-attachable image as a rendering
destination, this extension enables a form of "offscreen" rendering.
So, you would get your "non-real-time" rendering by rendering off-screen some scene that renders slower than 30fps, and then saving those results to some movie file or file-sequence format that can be played back at a later date.
I was wondering if it was faster to render a single quad the size of the window with a texture the size of a window than to draw the bitmap directly to the window using double buffering coupled with the platform specific way of drawing to a window.
The initial setup for textures tends to be relatively slow, but once that's done the drawing is quite fast -- in a typical case where graphics memory is available, it'll upload the texture to the memory on the graphics cards during initial setup, and after that, all the drawing will happen from there. At the same time, that initial upload will also typically include full a full mipmap down to 1x1 resolution, so you're uploading a bit more than just the full-resolution texture.
With platform specific drawing, you usually don't have quite as much work up-front. If only part of the bitmap is visible, only the visible part will be uploaded. If the bitmap is going to be scaled, it'll typically scale it on the CPU and send it to the card at the current scale (and never upload anything resembling a mipmap). OTOH, virtually every time something needs to be redrawn, it'll end up re-sending the bitmap data for the newly exposed area. It doesn't take much of that to lose the (often minor anyway) advantage of minimizing what was sent to start with.
Using textures is usually a lot faster, since most native drawing APIs aren't hardware accelerated.
It will very probably depend on the graphics card and driver.
I'm developing some C++ code that can do some fancy 3D transition effects between two images, for which I thought OpenGL would be the best option.
I start with a DIB section and set it up for OpenGL, and I create two textures from input images.
Then for each frame I draw just two OpenGL quads, with the corresponding image texture.
The DIB content is then saved to file.
For example one effect is to locate the two quads (in 3d space) like two billboards, one in front of the other(obscuring it), and then swoop the camera up, forward and down so you can see the second one.
My input images are 1024x768 or so and it takes a really long time to render (100 milliseconds) when the quads cover most of the view. It speeds up if the camera is far away.
I tried rendering each image quad as hundreds of individual tiles, but it takes just the same time, it seems like it depends on the number of visible textured pixels.
I assumed OpenGL could do zillions of polygons a second. Is there something I am missing here?
Would I be better off using some other approach?
Thanks in advance...
Edit :
The GL strings show up for the DIB version as :
Vendor : Microsoft Corporation
Version: 1.1.0
Renderer : GDI Generic
The Onscreen version shows :
Vendor : ATI Technologies Inc.
Version : 3.2.9756 Compatibility Profile Context
Renderer : ATI Mobility Radeon HD 3400 Series
So I guess I'll have to use FBO's , I'm a bit confused as to how to get the rendered data out from the FBO onto a DIB, any pointers (pun intended) on that?
It sounds like rendering to a DIB is forcing the rendering to happen in software. I'd render to a frame buffer object, and then extract the data from the generated texture. Gamedev.net has a pretty decent tutorial.
Keep in mind, however, that graphics hardware is oriented primarily toward drawing on the screen. Capturing rendered data will usually be slower that displaying it, even when you do get the hardware to do the rendering -- though it should still be quite a bit faster than software rendering.
Edit: Dominik Göddeke has a tutorial that includes code for reading back texture data to CPU address space.
One problem with your question:
You provided no actual rendering/texture generation code.
Would I be better off using some other approach?
The simplest thing you can do is to make sure your textures have sizes equal to power of two. I.e. instead of 1024x768 use 1024x1024, and use only part of that texture. Explanation: although most of modern hardware supports non-pow2 textures, they are sometimes treated as "special case", and using such texture MAY produce performance drop on some hardware.
I assumed OpenGL could do zillions of polygons a second. Is there something I am missing here?
Yes, you're missing one important thing. There are few things that limit GPU performance:
1. System memory to video memory transfer rate (probably not your case - only for dynamic textures\geometry when data changes every frame).
2. Computation cost. (If you write a shader with heavy computations, it will be slow).
3. Fill rate (how many pixels program can put on screen per second), AFAIK depends on memory speed on modern GPUs.
4. Vertex processing rate (not your case) - how many vertices GPU can process per second.
5. Texture read rate (how many texels per second GPU can read), on modern GPUs depends on GPU memory speed.
6. Texture read caching (not your case) - i.e. in fragment shader you can read texture few hundreds times per pixel with little performance drop IF coordinates are very close to each other (i.e. almost same texel in each read) - because results are cached. But performance will drop significantly if you'll try to access 100 randomly located texels for every pixels.
All those characteristics are hardware dependent.
I.e., depending on some hardware you may be able to render 1500000 polygons per frame (if they take a small amount of screen space), but you can bring fps to knees with 100 polygons if each polygon fills entire screen, uses alpha-blending and is textured with a highly-detailed texture.
If you think about it, you may notice that there are a lot of videocards that can draw a landscape, but fps drops when you're doing framebuffer effects (like blur, HDR, etc).
Also, you may get performance drop with textured surfaces if you have built-in GPU. When I fried PCIEE slot on previous motherboard, I had to work with built-in GPU (NVidia 6800 or something). Results weren't pleasant. While GPU supported shader model 3.0 and could use relatively computationally expensive shaders, fps rapidly dropped each time when there was a textured object on screen. Obviously happened because built-in GPU used part of system memory as video memory, and transfer rates in "normal" GPU memory and system memory are different.