OpenGL Positioning

[JesseGuarascia]

Gosh, I feel like an idiot asking questions like this on forums. Although, I feel like more of an idiot sitting here for hours not understanding something, and getting frustrated because I can't figure something out.

Alright, I recently started learning OpenGL, and I just moved on from SDL. I'm sort of used to absolute positioning, so I read up about glOrtho, which works fine for that sort of thing. Used that for a few hours, then read up on transformations and rotation as well as depth. So I learned about gluPerspective, and that's just totally too confusing for me. I learned how to draw objects in it from NeHe, and how to use all of the different little things like rotation and depth. I'm just so confused about how I'm supposed to draw objects in absolute positions, relative to the top-left of the screen. I like glOrtho using SDL positioning properties.

As far as I can tell, the only positioning ability using gluPerspective is floating point, which makes finding the top left corner kind of confusing. How would I approach something like this? I'm just drawing 2D objects (QUADS, TRIANGLES, LINES). I got interested seeing Elysian Shadows' uniform zoom and scale.

tl;dr: I guess what my real question is how would I approach positioning in a specific area (such as the top left corner) of a screen using gluPerspective? Thanks for any help you guys can give me

[TheBuzzSaw]

I strongly recommend that you try to let go of your attachment to fixed size pixels and the 0, 0 corner. Moving to 3D graphics involves a new way of thinking. Trust me: you will come to appreciate working in "object space" independent of the display resolution.

However, I will be a nice guy and try to answer your question. You could probably use glFrustum to force the effect you want. I still recommend you try getting used to having 0, 0 at the center of your scene.

[JesseGuarascia]

Thanks! All of the things I've read have told me do that too sadly. It's okay though, I'll maybe get used to it, I don't know. Anyways, thanks for the advice, I'll see if I can try to learn myself some stuff. I can use glTranslate to say, translate to the center of an object I'd like to draw, then move around from that. I understand totally fine now after a little testing. I'm just going to have to get used to how OpenGL center of the scene thing

[TheBuzzSaw]

I typed that last response on my iPad. XD

Here's the thing. There's more to it than just getting used to it because "everyone else does it". It really does help to start thinking of your scene in object space. By that, I mean you should stop thinking of your display as a grid of pixels (well, not entirely... just bear with me). Instead, you designate your own units of measurement and then set the screen based on how many units you need. For instance, let's say you decide that you need the screen to be 12 units tall. From there, it doesn't matter whether the screen is 800x600 or 1920x1080. You can draw your stuff based on the screen being 12 "units" tall, and your stuff will always end up in the same relative place on screen. If I'm not making sense, let me know, and I'll break it down further. You'll find it's really nice having 0, 0, 0 be at the center. It just makes the most sense once you stop operating off the corner of the screen.

[JesseGuarascia]

Hmm, that's a good way of thinking about it actually. So how would I go about setting my units? Right now, if I don't touch the Z-Layer, and use something like:

gluPerspective(90.f, width / height, 0.1f, 100.f);

I can't immediately draw unless I give a call to glTranslate, and move in on the Z-Layer a bit. I'm sure that doesn't matter, since each layer is probably sharing the units I created earlier I assume. So yeah, how would I set my own units? XD

[TheBuzzSaw]

If you plan on building a 2D game, I recommend using orthographic projection. It makes life much easier. If you plan on doing 3D in a perspective projection, yeah, you need to push the objects away a bit. Otherwise, they are sitting right on the camera, technically.

gluPerspective(60.0f, width / height, 1.0f, 100.f);

That's a good projection to have. Don't cram the near clipping plain near zero. Just keep it at 1.0.

[JesseGuarascia]

Okay, last thing (most likely). I want to use 3D, but just for like, displaying an object, and then zooming in and out whenever I feel like it. What's the best way to approach this? If I do orthographic is there a way to perhaps zoom in and out? Because glTranslatef doesn't do much of a zoom D:! (I hate being so helpless with graphic programming)

[TheBuzzSaw]

Correct. In an orthographic projection, the z-axis does not make anything bigger/smaller. For 2D games, emulate zooming by using scaling instead.

http://www.opengl.org/sdk/docs/man/xhtml/glScale.xml