Sign in to follow this  
kibokun

OpenGL OpenGL coordinate system

Recommended Posts

I'm very confused about why OpenGL defines the coordinates it uses when you call glVertex as offsets from the center of the screen. It's extremely counter-intuitive for just about everything I want to do. I need a bit of help understanding how to make this work logically with a grid-based collision system (and later, some sort of tree-based subdivision structure). Currently, the objects I want to display are just points drawn using glVertex2. Their position vectors are stored as offsets from the center of the screen, which is for some reason (0, 0). However, I want their coordinates in screen space. That is, the rect defined with an upper left point of (0,0) and the bottom right being (window width, window height). Is there a way to make OpenGL vertex calls work on regular coordinates or do I have to write some sort of function to translate them every time I want to use an object's position to draw a vertex? This would also be a solution (a computationally wasteful one, however.) to the problem of only having OpenGL offset coordinates when I wanted to check which grid cell an object resides in. Thanks.

Share this post


Link to post
Share on other sites
hmm, glOrtho definitely does what I want for the most part. However, what if I still want to have depth? Should I be using glVertex3f(...) instead of glVertex2f and just keep the Z component at zero?

I ask because I might want to zoom the camera back to show a larger landscape, but still only operate in 2 dimensions for everything else.

This seems to be more logical, as it's only the illusion of 2d.

Share this post


Link to post
Share on other sites
Quote:
Original post by kibokun
hmm, glOrtho definitely does what I want for the most part. However, what if I still want to have depth? Should I be using glVertex3f(...) instead of glVertex2f and just keep the Z component at zero?


This will just make it possible to "occlude" something because higher Z will overdraw lower Z. With glOrtho you are literally mapping XY coordinates to screen space. You could zoom out by scaling your coordinates or changing the values you pass to glOrtho.

If you want 3D then stick to perspective mode. The "center of the screen" thing is an illusion. You are specifying ModelView coordinates but the View is the identity matrix. So you start at the origin looking down the negative Z axis. Try adding some glTranslate / glRotate calls before your glVertex calls and observe how the "camera" changes.

By the way, all the glVertex etc calls are now deprecated (but it's still ok to use them if you want to).

Share this post


Link to post
Share on other sites
Quote:
Original post by kibokun
Is there a way to make OpenGL vertex calls work on regular coordinates ...
Why do you think that the one co-ordinate system is regular and the other is not? In fact none system is superior to another in general. Its just more convenient to do some task in the one system instead of the other.

Quote:
Original post by kibokun
... or do I have to write some sort of function to translate them every time I want to use an object's position to draw a vertex? This would also be a solution (a computationally wasteful one, however.) to the problem of only having OpenGL offset coordinates when I wanted to check which grid cell an object resides in.
OpenGL does a transformation anyway, and that is much more complex that just a single 2D translation (i.e. it is a 4D matrix by vector product).

Quote:
Original post by kibokun
... However, what if I still want to have depth? Should I be using glVertex3f(...) instead of glVertex2f and just keep the Z component at zero?
Read this:
Quote:
Manpage of glVertex
When only x and y are specified, z defaults to 0 and w defaults to 1. When x, y, and z are specified, w defaults to 1.


Quote:
Original post by kibokun
I ask because I might want to zoom the camera back to show a larger landscape, but still only operate in 2 dimensions for everything else.
Zoom has nothing to do with depth; it cannot even be faked by moving the camera because you utilize orthographic projection. Instead, adapt glOrtho's parameters accordingly.

Share this post


Link to post
Share on other sites
I wouldn't suggest to use scale (maybe it's just me, but simply I don't like scaling). If you give greater values to glOrtho, that means more stuff can "fit" into it, thus to the screen, so there you have zooming out. Twice as big values, you zoomed out with two. It's that simple.

Share this post


Link to post
Share on other sites
Thanks for the replies. Very informative. :)

I think I'm going to stay with gluPerspective because it's more natural for me. However, I'm still not understanding how I can accomplish this:

When a user clicks on a point on the screen, a particle should be created at that X and Y position. However, the X and Y positions I get from the SDL event are on the range of 0-800 for X and 0-600 for Y.

I'd like to find the corresponding X and Y positions I need to pass to glVertex to place a vertex under the mouse cursor.

Thanks!

Share this post


Link to post
Share on other sites
If you use ortho mode it is fairly straight forward, if you use projection mode it's a little more fiddly as you have to transform the screen space mouse coordinates into world space but not a major ball ache with gluUnproject.

Share this post


Link to post
Share on other sites
The basic idea is that you unproject the mouse coordinates twice to get a ray, then intersect that ray with whatever plane you want the particle at (let's say y=0, meaning ground plane).

Share this post


Link to post
Share on other sites
After a lot of struggle with UnProject, I switched to glOrtho. It seems to work great for what I need, except zooming in and out. This was trivial with gluPerspective. :(


My problem is when I zoom out, it's as if the vanishing point in gluPerspective were the top left corner of the window. Everything shrinks into that corner when I really just want to zoom out and have objects stay in their place on the screen. What do I have to do to accomplish this?

Here's my function to change the glOrtho arguments



void GameplayState::SetProjection()
{
GLint viewport[4];
glGetIntegerv(GL_VIEWPORT, viewport);

int zoomedWidth = viewport[2] / (2.0f*this->zoomFactor);
int zoomedHeight = viewport[3] / (2.0f*this->zoomFactor);

glOrtho(0, zoomedWidth, zoomedHeight, 0, 0, 1);

glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
}

Share this post


Link to post
Share on other sites
You can either translate the objects to the new position or, ironically, make glOrtho work with the origin being at the center of the screen. Something like glOrtho(-zoomedWidth/2, zoomedWidth/2, zoomedHeight/2, -zoomedHeight/2, 0, 1); should do the trick.

Share this post


Link to post
Share on other sites
Quote:
Original post by kibokun
After a lot of struggle with UnProject, I switched to glOrtho. It seems to work great for what I need, except zooming in and out. This was trivial with gluPerspective. :(


My problem is when I zoom out, it's as if the vanishing point in gluPerspective were the top left corner of the window. Everything shrinks into that corner when I really just want to zoom out and have objects stay in their place on the screen. What do I have to do to accomplish this?

Here's my function to change the glOrtho arguments



void GameplayState::SetProjection()
{
GLint viewport[4];
glGetIntegerv(GL_VIEWPORT, viewport);

int zoomedWidth = viewport[2] / (2.0f*this->zoomFactor);
int zoomedHeight = viewport[3] / (2.0f*this->zoomFactor);

glOrtho(0, zoomedWidth, zoomedHeight, 0, 0, 1);

glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
}
glOrtho() taks the left, right, bottom, and top bounds of the visible region as arguments. In your case, the view remains achored at a corner because you always make the top-left corner (0, 0). If you don't want the view to be anchored at the top-left corner, then don't achor the view at the top-left corner :)

It might help to think of your orthographic projection in terms of a center (focal) point, an aspect ratio, and a zoom value. Here's an example of how you might derive the glOrtho() arguments from these values:
float yExtent = zoom;
float xExtent = yExtent * aspectRatio;
float left = center.x - xExtent;
float right = center.x + xExtent;
// You can swap these if you want +y to go from top to bottom:
float bottom = center.y - yExtent;
float top = center.y + yExtent;
You can then move your virtual camera around by modifying its 'center' value, and zoom in and out by modifying the 'zoom' value.

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

Sign in to follow this  

  • Forum Statistics

    • Total Topics
      627773
    • Total Posts
      2979011
  • Similar Content

    • By lonewolff
      Hi guys,
      With OpenGL not having a dedicated SDK, how were libraries like GLUT and the likes ever written?
      Could someone these days write an OpenGL library from scratch? How would you even go about this?
      Obviously this question stems from the fact that there is no OpenGL SDK.
      DirectX is a bit different as MS has the advantage of having the relationship with the vendors and having full access to OS source code and the entire works.
      If I were to attempt to write the most absolute basic lib to access OpenGL on the GPU, how would I go about this?
    • By DelicateTreeFrog
      Hello! As an exercise for delving into modern OpenGL, I'm creating a simple .obj renderer. I want to support things like varying degrees of specularity, geometry opacity, things like that, on a per-material basis. Different materials can also have different textures. Basic .obj necessities. I've done this in old school OpenGL, but modern OpenGL has its own thing going on, and I'd like to conform as closely to the standards as possible so as to keep the program running correctly, and I'm hoping to avoid picking up bad habits this early on.
      Reading around on the OpenGL Wiki, one tip in particular really stands out to me on this page:
      For something like a renderer for .obj files, this sort of thing seems almost ideal, but according to the wiki, it's a bad idea. Interesting to note!
      So, here's what the plan is so far as far as loading goes:
      Set up a type for materials so that materials can be created and destroyed. They will contain things like diffuse color, diffuse texture, geometry opacity, and so on, for each material in the .mtl file. Since .obj files are conveniently split up by material, I can load different groups of vertices/normals/UVs and triangles into different blocks of data for different models. When it comes to the rendering, I get a bit lost. I can either:
      Between drawing triangle groups, call glUseProgram to use a different shader for that particular geometry (so a unique shader just for the material that is shared by this triangle group). or
      Between drawing triangle groups, call glUniform a few times to adjust different parameters within the "master shader", such as specularity, diffuse color, and geometry opacity. In both cases, I still have to call glBindTexture between drawing triangle groups in order to bind the diffuse texture used by the material, so there doesn't seem to be a way around having the CPU do *something* during the rendering process instead of letting the GPU do everything all at once.
      The second option here seems less cluttered, however. There are less shaders to keep up with while one "master shader" handles it all. I don't have to duplicate any code or compile multiple shaders. Arguably, I could always have the shader program for each material be embedded in the material itself, and be auto-generated upon loading the material from the .mtl file. But this still leads to constantly calling glUseProgram, much more than is probably necessary in order to properly render the .obj. There seem to be a number of differing opinions on if it's okay to use hundreds of shaders or if it's best to just use tens of shaders.
      So, ultimately, what is the "right" way to do this? Does using a "master shader" (or a few variants of one) bog down the system compared to using hundreds of shader programs each dedicated to their own corresponding materials? Keeping in mind that the "master shaders" would have to track these additional uniforms and potentially have numerous branches of ifs, it may be possible that the ifs will lead to additional and unnecessary processing. But would that more expensive than constantly calling glUseProgram to switch shaders, or storing the shaders to begin with?
      With all these angles to consider, it's difficult to come to a conclusion. Both possible methods work, and both seem rather convenient for their own reasons, but which is the most performant? Please help this beginner/dummy understand. Thank you!
    • By JJCDeveloper
      I want to make professional java 3d game with server program and database,packet handling for multiplayer and client-server communicating,maps rendering,models,and stuffs Which aspect of java can I learn and where can I learn java Lwjgl OpenGL rendering Like minecraft and world of tanks
    • By AyeRonTarpas
      A friend of mine and I are making a 2D game engine as a learning experience and to hopefully build upon the experience in the long run.

      -What I'm using:
          C++;. Since im learning this language while in college and its one of the popular language to make games with why not.     Visual Studios; Im using a windows so yea.     SDL or GLFW; was thinking about SDL since i do some research on it where it is catching my interest but i hear SDL is a huge package compared to GLFW, so i may do GLFW to start with as learning since i may get overwhelmed with SDL.  
      -Questions
      Knowing what we want in the engine what should our main focus be in terms of learning. File managements, with headers, functions ect. How can i properly manage files with out confusing myself and my friend when sharing code. Alternative to Visual studios: My friend has a mac and cant properly use Vis studios, is there another alternative to it?  
    • By ferreiradaselva
      Both functions are available since 3.0, and I'm currently using `glMapBuffer()`, which works fine.
      But, I was wondering if anyone has experienced advantage in using `glMapBufferRange()`, which allows to specify the range of the mapped buffer. Could this be only a safety measure or does it improve performance?
      Note: I'm not asking about glBufferSubData()/glBufferData. Those two are irrelevant in this case.
  • Popular Now