Sign in to follow this  
Taylor001

OpenGL OpenGL camera class

Recommended Posts

Taylor001    125
Hi to all, it's my first post here! I need some help with the camera class I'm making. In order to describe the camera, I use a vector to store the camera's position and three vectors for the right, up, and view directions. To calculate each of them I do this: 1. Rotate the view and the right vector by an "alpha" amount about the world's up vector (0, 1, 0) 2. Derive a matrix for rotating about the computed right vector 3. Rotate both the view and up vector multiplying them by this matrix Once I get the three basis vectors, I construct a column-major matrix placing the vectors in this way: [ right.x up.x view.x ] [ right.y up.y view.y ] [ right.z up.z view.z ] Then, I call glLoadMatrixf() to use it. The problem is that the camera doesn't face well in some situations, like when I turn left by 90° and look up or down. Actually, I don't think I'm doing something wrong in the calculation of the basis vectors, instead, I think there's something wrong in placing them into the matrix. Indeed, if I transpose the above matrix, i.e. if I put basis vectors in this way: [ right.x right.y right.z ] [ up.x up.y up.z ] [ view.x view.y view.z ] it seems to work just fine... BUT, shouldn't the basis vectors be positioned like in the first form, i.e. as column vectors as long as I'm using coloumn-major matrices (and OpenGL as well)? Thanks in advance!

Share this post


Link to post
Share on other sites
jyk    2094
Quote:
BUT, shouldn't the basis vectors be positioned like in the first form, i.e. as column vectors as long as I'm using coloumn-major matrices (and OpenGL as well)?
Thanks in advance!
That's correct. However, if your intention is for your camera to function as an actual 'camera', you would typically invert the matrix before uploading it to OpenGL (so that it will have the effect of a 'view' matrix). The result of the extra transpose you have in there is similar to the result of this inversion, which is probably why the camera behavior seems 'less incorrect' with the extra transpose there.

In short, try removing the 'extra' transpose and replacing it with a proper matrix inversion.

Share this post


Link to post
Share on other sites
Taylor001    125
So, what I should do is simply load the 'inverse' of the matrix I'm currently loading and it's all ok. To derive the inverse of my matrix I simply transpose its rotation section, negate all of the translation part and leave the last row to (0 0 0 1).

Now the camera works as it should but I still have to figure out 'why'. I just cannot imagine what's exactly happening.

Share this post


Link to post
Share on other sites
jyk    2094
Quote:
To derive the inverse of my matrix I simply transpose its rotation section, negate all of the translation part and leave the last row to (0 0 0 1).
Actually, that's not quite right. Are you sure it's working correctly in all cases?

In any case, I'd recommend looking online for a general 4x4 matrix inversion function and just using that. (You can in fact treat inversion of rigid-body transform matrices as a special case, but in the case of a 'view' matrix I don't know that it's really worth the effort.)
Quote:
Now the camera works as it should but I still have to figure out 'why'. I just cannot imagine what's exactly happening.
Don't have time for a full explanation, but here are a couple of hints:

1. In general, the inverse of a transform matrix represents a transform that does the 'opposite' of what the original transform does.

2. If a given transform transforms geometry from local to world space, the inverse of that transform will transform geometry from world to local space.

3. In the graphics pipeline, prior to projection but after all other transforms have been applied, we want to transform the geometry into the local space of the camera (which is where the 'projection' part of the pipeline expects it to be). This is what the inverse camera matrix does.

Share this post


Link to post
Share on other sites
Taylor001    125
Quote:
Actually, that's not quite right. Are you sure it's working correctly in all cases?
Yes, I checked it and I'm pretty sure it works in all cases.
Quote:
In any case, I'd recommend looking online for a general 4x4 matrix inversion function and just using that. (You can in fact treat inversion of rigid-body transform matrices as a special case, but in the case of a 'view' matrix I don't know that it's really worth the effort.)
I'm reading a book on this and soon will implement a function to substitute that bad matrix I'm deriving by hand. :)

However, I'd like to thank you for your time and for your hints that were at least illuminating. Slowly I'm understanding more and more about this and it's also thanks to people like you!

Share this post


Link to post
Share on other sites
Taylor001    125
I made a mistake in explaining how I derive the working matrix! I said I builded a transposed rotation matrix with a negated translation part, but, actually I didn't do this. Instead, what I do is to build a transposed rotation matrix and a _separate_ translation matrix negating its translation part. Then I concatenate them to form the final matrix to upload to OpenGL. It's for this that it works well the same, I think.

[Edited by - Taylor001 on December 18, 2008 4:27:52 PM]

Share this post


Link to post
Share on other sites
jyk    2094
Quote:
Original post by Taylor001
I made a mistake in explaining how I derive the working matrix! I said I builded a transposed rotation matrix with a negated translation part, but, actually I didn't do this. Instead, what I do is to build a transposed rotation matrix and a _separate_ translation matrix negating its translation part. Then I concatenate them to form the final matrix to upload to OpenGL. It's for this that it works well the same, I think.
Yes, that sounds right (assuming you're concatenating the transforms in the right order, which it sounds like you are).

Share this post


Link to post
Share on other sites
Taylor001    125
Ok, I just implemented the inverse function. Now I load both the rotation part and the translation part in a single matrix, make the inverse and upload to OpenGL. Seems to work perfectly. Thanks a lot!

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  

  • Partner Spotlight

  • Similar Content

    • By pseudomarvin
      I assumed that if a shader is computationally expensive then the execution is just slower. But running the following GLSL FS instead just crashes
      void main() { float x = 0; float y = 0; int sum = 0; for (float x = 0; x < 10; x += 0.00005) { for (float y = 0; y < 10; y += 0.00005) { sum++; } } fragColor = vec4(1, 1, 1 , 1.0); } with unhandled exception in nvoglv32.dll. Are there any hard limits on the number of steps/time that a shader can take before it is shut down? I was thinking about implementing some time intensive computation in shaders where it would take on the order of seconds to compute a frame, is that possible? Thanks.
    • By Arulbabu Donbosco
      There are studios selling applications which is just copying any 3Dgraphic content and regenerating into another new window. especially for CAVE Virtual reality experience. so that the user opens REvite or CAD or any other 3D applications and opens a model. then when the user selects the rendered window the VR application copies the 3D model information from the OpenGL window. 
      I got the clue that the VR application replaces the windows opengl32.dll file. how this is possible ... how can we copy the 3d content from the current OpenGL window.
      anyone, please help me .. how to go further... to create an application like VR CAVE. 
       
      Thanks
    • By cebugdev
      hi all,

      i am trying to build an OpenGL 2D GUI system, (yeah yeah, i know i should not be re inventing the wheel, but this is for educational and some other purpose only),
      i have built GUI system before using 2D systems such as that of HTML/JS canvas, but in 2D system, i can directly match a mouse coordinates to the actual graphic coordinates with additional computation for screen size/ratio/scale ofcourse.
      now i want to port it to OpenGL, i know that to render a 2D object in OpenGL we specify coordiantes in Clip space or use the orthographic projection, now heres what i need help about.
      1. what is the right way of rendering the GUI? is it thru drawing in clip space or switching to ortho projection?
      2. from screen coordinates (top left is 0,0 nd bottom right is width height), how can i map the mouse coordinates to OpenGL 2D so that mouse events such as button click works? In consideration ofcourse to the current screen/size dimension.
      3. when let say if the screen size/dimension is different, how to handle this? in my previous javascript 2D engine using canvas, i just have my working coordinates and then just perform the bitblk or copying my working canvas to screen canvas and scale the mouse coordinates from there, in OpenGL how to work on a multiple screen sizes (more like an OpenGL ES question).
      lastly, if you guys know any books, resources, links or tutorials that handle or discuss this, i found one with marekknows opengl game engine website but its not free,
      Just let me know. Did not have any luck finding resource in google for writing our own OpenGL GUI framework.
      IF there are no any available online, just let me know, what things do i need to look into for OpenGL and i will study them one by one to make it work.
      thank you, and looking forward to positive replies.
    • By fllwr0491
      I have a few beginner questions about tesselation that I really have no clue.
      The opengl wiki doesn't seem to talk anything about the details.
       
      What is the relationship between TCS layout out and TES layout in?
      How does the tesselator know how control points are organized?
          e.g. If TES input requests triangles, but TCS can output N vertices.
             What happens in this case?
      In this article,
      http://www.informit.com/articles/article.aspx?p=2120983
      the isoline example TCS out=4, but TES in=isoline.
      And gl_TessCoord is only a single one.
      So which ones are the control points?
      How are tesselator building primitives?
    • By Orella
      I've been developing a 2D Engine using SFML + ImGui.
      Here you can see an image
      The editor is rendered using ImGui and the scene window is a sf::RenderTexture where I draw the GameObjects and then is converted to ImGui::Image to render it in the editor.
      Now I need to create a 3D Engine during this year in my Bachelor Degree but using SDL2 + ImGui and I want to recreate what I did with the 2D Engine. 
      I've managed to render the editor like I did in the 2D Engine using this example that comes with ImGui. 
      3D Editor preview
      But I don't know how to create an equivalent of sf::RenderTexture in SDL2, so I can draw the 3D scene there and convert it to ImGui::Image to show it in the editor.
      If you can provide code will be better. And if you want me to provide any specific code tell me.
      Thanks!
  • Popular Now