Sign in to follow this  
shafiee01

OpenGL pixel to centimeter

Recommended Posts

shafiee01    122
Hi I have a simple problem but i don't know how to solve it! i am developing a simulator with opengl and everything is ok but i need to measure my distances in centimeter but in opengl everything is pixel so i want to know how to convert them at different resolution and monitors! thanks a lot!

Share this post


Link to post
Share on other sites
Brother Bob    10344
If you refer to the mapping of pixels on the screen, to the corresponding size in real world units as appearing on the screen, then you need to look into your platform API and search for functions to determine the physical size of the screen. It is not related to OpenGL in any way though.

Share this post


Link to post
Share on other sites
Evil Steve    2017
Under Windows there's not really any way to do this - the user could be running at 1024x768 on a 14 inch monitor and get 1cm = 50 pixels, or they might be running on a 52 inch screen where 1cm = 2 pixels, and there's no API to tell you the physical of the screen.

Share this post


Link to post
Share on other sites
haegarr    7372
Quote:
Original post by shafiee01
...but in opengl everything is pixel...
Nope. The pixels are just the last step when computing distances:

1st you have to decide on the world length unit. This is freely chooseable, let's say 1m. This is to be considered when importing meshes. E.g. the mesh of a human should then be scaled on import so that it is approx. 1.7 units high.

2nd you have the extent of what I call the "nominal view", i.e. the size of the view plane in world units. Unfortunately, the view plane's size is constant only for parallel projections; it is depth dependent for perspective projections.

3rd you have to consider the angle with which the projectors hit the view plane. The factor is sin( α ) where α denotes the angle in the respective dimension (w.r.t. the camera space). E.g. for orthogonal projections the factor is 1.

4th you have a scaling factor incorporated into the VIEW portion of the MODELVIEW matrix. This scaling denotes the zoom of the virtual camera.

The above defines the viewing volume. Now comes the screen into play:

5th the view volume is projected onto a plane, and the plane is sampled. Each sample is then typically displayed on the screen with a 1:1 relationship to pixels. That is what I call the actual view. It has a (physical) size corresponding to the amount of pixel columns/rows multiplied by the screen resolution.

That said, you can sensefully measure distances just under special conditions. E.g. using parallel projection, or else measuring only between points that have the same depth _and_ this depth is known (e.g. on the view plane when those is defined to do a 1:1 projection).

It can be senseful to define a viewing system based on the above. However, IMHO the only senseful way to measure distances is to let the user pick 2 points (typically vertices) and to compute the distance in 3D, multiply that with the length unit chosen under point 1, and display that as text.

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