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goowik

OpenGL
Rendering possibilities

11 posts in this topic

Hi,

I've started learning OpenGL a few days back and love it [img]http://public.gamedev.net//public/style_emoticons/default/smile.png[/img]
It's been a few months to almost a year I've been creating games in 2D, time to change!

As of now I always created my primitive objects with an immediate rendering coding
For example:
[CODE]
glBegin(GL_TRIANGLES);
glColor3f(1, 0, 0);
glVertex2f(-0.5f, -0.5f);
glColor3f(0, 1, 0);
glVertex2f(0.5f, -0.5f);
glColor3f(0, 0, 1);
glVertex2f(0.5f, 0.5f);
glEnd();
[/CODE]

After reading some articles on performances I found out there are several more rendering "patterns". There is:[list]
[*]Display lists
[*]Vertex arrays
[*]Vertex buffer object
[*]and Immediate
[/list]
Does it come to personal preferences when choosing one of them? Or is there a major difference?

Also let's say you have a cube. How can you cull 3 of the invisible sides and make it change each time you move around it.
Is this coding different per rendering pattern?

Kind regards
Peter Edited by goowik
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You are using legacy OpenGL. It is fine for testing small simple things, but not good enough for "real" applications. And it is usually not feasible to start with legacy, and then upgrade, because it is done differently.

Have a look at [url="http://www.arcsynthesis.org/gltut/"]Learning Modern 3D Graphics Programming[/url], and you will quickly learn the modern way of doing it.

Unfortunately, most of the tutorials "out there" are done in the old way.
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@lardpensjo:
Just so I understand better: By Legacy OpenGL you mean the way of writing code? Not the library I'm using?
Thanks a bunch for the link! Guess I have some work to do testing out :)

@mark ds:
okay, I guess I forgot to mention that I'm using [url="http://www.lwjgl.org/index.php"]LWJGL [/url](The Lightweight Java Game Library). But I guess the OpenGL code isn't much different from c++.
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@mhagain:
Thanks for the detailed explanation. But after searching OpenGL's wiki I found the [url="http://www.opengl.org/wiki/Vertex_Buffer_Object"]Vertex Buffer Object page[/url].


Unfortunatly the following is written:
[quote]Legacy Note: Versions of OpenGL prior to 3.0 allowed the use of client-side data for vertex rendering, but GL 3.0 deprecated this. Core GL 3.1 and greater forbid client-side vertex data, so VBOs are the only means for rendering.[/quote]

And they state offcourse that it is recommended that we do not use any of these functionality in our programs.

So I have a two questions about this:[list=1]
[*]What is the meaning of Legacy? as 'lardpensjo' stated.
[*]What do they mean by client-side data? Isn't everything client sided?
[/list]
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[quote name='goowik' timestamp='1341341075' post='4955397']
So I have a two questions about this:[list=1]
[*]What is the meaning of Legacy? as 'lardpensjo' stated.
[*]What do they mean by client-side data? Isn't everything client sided?
[/list]
[/quote]
Legacy refers to the old OpenGL API up to version 2.1. Much of OpenGL 2.1 was removed from the API when 3.0 was introduced (there was a combatibility mode for some versions, but that compatiblity mode has been removed as well in 3.3), incluiding things such as display lists and client-side vertex arrays. Legacy refers to the now-deprecated API of version 2.1 and earlier.

Client-side and server-side in this context refers to the application (the client) and the OpenGL implementation/driver (the server). These are typically on the same computer (you run the application on the same computer you have your graphics card on), but that does not have to be the case. Especially on unix-platforms and their windowing systems, you can basically have the application run on one computer and have the display on another computer.

Client-side data in this context means the data is stored in memory managed by the application. For example, when you allocate the memory with malloc or new, or store the data in an std::vector). Server-side data means that the data is stored in memory managed by OpenGL. For example, texture data stored in a texture object with glTexImage or vertex arrays stored in a VBO are both stored in memory managed by OpenGL. They don't have to be physically stored on the graphics card, only that the memory is managed by OpenGL.

Server-side data is the only way to store any data in modern OpenGL. You may have to load your vertex arrays into your own memory before uploading them to your buffer objects of course, but you cannot use the vexrtex arrays to draw something until it has been uploaded into VBOs. Server-side is mandatory for everything involving user data at the moment, not just vertex arrays.
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If you have the choice, consider the API that is geared for the future of OpenGL. On mobile devices that would be OpenGL ES 2.0. Even if OpenGL 4.2 still supports the legacy api, you should try to avoid it if you can...
That being said, using immediate mode is still ok for quick testing and learning. But for the long run, and production code, avoid legacy API.
This page has some info about OpenGL Core profile: http://www.opengl.org/wiki/Core_And_Compatibility_in_Contexts

Look for glCullFace to find out how you can cull the invisible faces of your cube. glCullFace will always work no matter the rendering API you use.
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@Brother Bob:
Okay now I get it! So most of the code is backwards compatible till OpenGL 3.0.
Just out of curiosity, client/server side is this applicable to most graphic libraries (such as directX)?

@CodedVentures:
K so that clears most of it. To recap you simply have 2 "packages" (if I may call it this way):[list]
[*]The Core package
[*]The Compatibility (which was introduced in 3.2)
[/list]
Will the core always be the same? As all deprecated methods were removed?


EDIT:
I also was looking for a decent book to buy/rent and came across the "[i][b]OpenGL Superbible[/b][/i]"
But on [url="http://www.amazon.com/OpenGL-SuperBible-Comprehensive-Tutorial-Reference/dp/0321712617"]Amazon[/url] it does seem to have a low score (3.5/5) and the reviews are very various. Edited by goowik
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[quote name='goowik' timestamp='1341384981' post='4955542']
@Brother Bob:
Okay now I get it! So most of the code is backwards compatible till OpenGL 3.0.
[/quote]
That is correct. The compatibility mode of the modern API lets you extent the backward compatibility of the legacy API to version 3.1 even.

The modern API is mostly backward compatible as well, although there are some changes that makes it not. For example, vertex array objects (VAO) are not required in version 3, but are in version 4. That means that you cannot run a version 3 compatible program on a version 4 context. But that change is trivial; just use VAO in version 3 as well and you will have no problems with version 4.

[quote name='goowik' timestamp='1341384981' post='4955542']
Just out of curiosity, client/server side is this applicable to most graphic libraries (such as directX)?
[/quote]
I have no idea whether Direct3D makes the difference explicit in some way or not. I would, however, guess that pretty much the same requirements on memory management applies to both Direct3D and OpenGL: manual memory management is not possible, you have to let the API handle your resources.

[quote name='goowik' timestamp='1341384981' post='4955542']
@CodedVentures:
K so that clears most of it. To recap you simply have 2 "packages" (if I may call it this way):[list]
[*]The Core package
[*]The Compatibility (which was introduced in 3.2)
[/list]
Will the core always be the same? As all deprecated methods were removed?
[/quote]
The core changes, but (with few exceptions, see for example my comment above on VAO in version 3 and 4) new stuff are only added. What works on an earlier version typically works on a later version as well. Only exception of course is the major change from the legacy API to the modern API.

The compatibility mode was introduced in 3.0 though. From version 3.2 and onwards, compatibility mode is not available. Thus, it is only available for the modern API for versions 3.0 and 3.1.
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[quote name='goowik' timestamp='1341384981' post='4955542']
I also was looking for a decent book to buy/rent and came across the "OpenGL Superbible"
But on Amazon it does seem to have a low score (3.5/5) and the reviews are very various.
[/quote]
I have that book, and I think the weakness is that it is that the examples are based on a home made support library. The disadvantage of that is for beginners, that do want simple examples where you can see the whole solution on the same page. The advantage is for the intermediate or advanced reader, that no longer cares about the basics but want to learn and understand advanced concepts.

If you have understood the basics, then the book is excellent.

Actually, a member of the forum, [url="http://www.gamedev.net/user/158805-japro/"]japro[/url], recently created basic minimal examples, perfect for beginners to start with. See [url="https://github.com/progschj/OpenGL-Examples"]https://github.com/progschj/OpenGL-Examples[/url].
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