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This is a question about 3D programming in general, but I'm learning OpenGL if that makes the answer any different. What I wonder is if all of the work in displaying a image always has to start from scratch for each new frame, or if there's some way to save intermediate data that could be reused when rendering the next frame, instead of having to be recomputed? Let's say you're standing right next to a mountain, then the stuff on the other side of the mountain are occluded by the mountain, there could be a lot of stuff on the other side of the mountain that simply doesn't have to be rendered because it can't be seen. Now assume that your character can't walk particularly fast, then there's no way that the stuff on the other side of the mountain could be visible already in the next frame, or maybe not even the next 100 frames. Is it possible to avoid having to do the same occlusion check in each frame?
The problem you're referring to is called "hidden surface removal" and "occlusion culling".
In realtime graphics it's usual to rerender each frame from scratch. However every good renderer will omit all the things that are definitively not visible. There are various algorithms for this.
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I am aware that similar questions have been asked many times before. However, this is a different case.
I am writing a game in C++ using SDL2. Currently, all objects draw themselves. They do this because they all draw slightly differently.
For instance, a Button contains a rectangle, drawn with SDL_RenderFillRect();
Buttons also contain text, which are drawn using SDL_RenderCopy(), which takes a texture generated by SDL_TTF as a parameter.
Additionally, a MapView widget (basically a grid that can load a tilemap) draws the grid out using a 'for' loop containing horizontal and vertical SDL_RenderDrawLine() calls.
Finally, the tiles themselves are stored as textures, drawn using SDL_RenderCopy().
I understand that it is generally preferable to NOT have objects draw themselves. However, because there is so much variation in how the objects are drawn, I'm not sure of another way!
I thought it might be possible to have a GetTexture() function for each object, and the ones using textures could simply 'return texture', while the geometric objects could generate a texture. This gets complicated with my MapView object, because the grid is constantly updated when the user navigates around the game world (an offset value is changed and the grid is redrawn when moved).
Like so many questions of this type the answer is: it depends on your program.
If you are only every going to draw it the same way using SDL, then no reason why not. Another alternative might be to have a specific rendering class for each object, but that's doubling the effort. Having all your rendering code in a single class or function works fine too, but it gets big and complicated fast.
It's a judgement call based on the complexity of your code and what you want to do with it in the future, and my advice is to choose the simplest solution. As long as you've considered the potential downsides, you can make an educated decision.
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I've been using pixels as units for aspects of my game such as movement, but I've been told that this is a bad practice. However, I've never seen a good explanation on what I'm supposed to do instead. Can somebody provide a good explanation on how to handle units in my games?
It doesn't matter what units you use; they can be completely arbitrary. The only thing that you need to do is make sure that they are not fixed to the screen pixels, because you later find out you want to change the scale of things that are displayed. It's OK if the conversion factor happens to be 1; just make sure that the conversion exists, so that you can change it if you have reason to later.
(And, as a practical matter, don't make the conversion exactly 1 because that hides bugs if you forgot to convert in one place.)
For 3D realistic games, a common unit is "1 meter". Real world units don't matter, but the idea is to use a unit that is similar to the size of objects in your world.
For tile-based or voxel-based games, a common unit is the width of one tile. This also allows you to omit some conversions, but you're much less likely to have a problem tying to tiles than to pixels because tiles affect the game rules anyway.
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I'm in the middle of implementing deferred shading in an engine I'm working on, and now I have to make a decision on whether to use a full RGB32F texture to store positions, or reconstruct it from the depth buffer. So it's basically a RGB32F texel fetch vs a matrix vector multiplication in the fragment shader. Also the trade between memory and extra ALU operations.
Please direct me to useful resources and tell me your own experience with the subject.
In my opinion it is preferable to recalculate the position from depth. This is what I do in my deferred engine. The recalculation is a fast enough to not even show up when I've been profiling the render loop. And that (virtually no performance impact) compared to ~24MB of extra video memory usage (for a 1920x1080 texture) was an easy choice for me.
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I have tried blending and this seems to provide a basic glass effect but I feel there must be a better way to generate a glass or ice style effect. What would people suggest ? Is there something that can be done with semi-transparent textures ?
This is a very broad and complex question and the answer entirely depends on what kind of result (in terms of realism etc.) you are trying to get, what kind of lighting you want etc. Most of these effects, and materials in general, are the domain of shaders. A lot can be achieved with choosing the right textures with the right material parameters - again depending on what you consider an acceptable result.
GPU Gems book has a chapter on glass simulation (see 19.3.2):
GPU Gems 2 - Generic Refraction Simulation
When it comes to ice, there are again a ton of different things to consider depending on the complexity you want - see this answer here:
How to render realistic ice?
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I have a model designed in Blender that i am going to render, in my game engine built using opengl, after exporting it to collada. The model in blender is divided into groups. These groups contains vertices normals and textures with there own index. is it a good thing to do if i rendered each group as a separate vbo or should i render the whole model as a single vbo?
Rules of thumb are:
always do as few OpenGL state changes as possible;
always talk to OpenGL as little as possible.
The main reason being that talking to the GPU is costly. The GPU is much happier if you give it something it'll take a long time to do and then don't bother it again while it's working. So it's likely that a single VBO would be a better solution, unless it would lead to a substantial increase in the amount of storage you need to use and hence run against caching elsewhere.