Still celebrating National IT Professionals Day with 3 months of free Premium Membership. Use Code ITDAY17

x
Solved

# unexpected depth value returned by gluUnProject

Posted on 2004-09-09
Medium Priority
428 Views
I've found the resources on the net to use gluUnProject() to get the world (x,y,z) values given a screen coordinate. The code seems to work, but I'm a little confused by the z value returned. The application renders a triangle strip in which all z values are -3.0f, but the z value returned from gluUnProject is -1.0f.

The application sets the viewport with

gluPerspective(37.0, (GLfloat)w/(GLfloat)h, 1, 20.0);

The vertices get setup with
glBegin(GL_TRIANGLE_STRIP);
glVertex3f( xstep, ystep, -3.0f);
...

Any idea why the z value isn't -3.0f?
0
Question by:ctuffli
[X]
###### Welcome to Experts Exchange

Add your voice to the tech community where 5M+ people just like you are talking about what matters.

• Help others & share knowledge
• Earn cash & points
• 3

LVL 17

Expert Comment

ID: 12018593
From :
>>>>
gluUnProject() does the opposite. It takes an XYZ window coordinate and returns the back-transformed XYZ object coordinate equivalent.

The concept of window space Z is often confusing. It's the depth buffer value expressed as a GLdouble in the range 0.0 to 1.0. Assuming a default glDepthRange(), a window coordinate with a Z value of 0.0 corresponds to an eye coordinate located on the zNear clipping plane. Similarly, a window space Z value of 1.0 corresponds to an eye space coordinate located on the zFar plane. You can obtain any window space Z value by reading the depth buffer with glReadPixels().
<<<<

So, basically depending upon the screen [z] value, you'll get back different world [z] values.  If you have a particular x,y on the screen that you want to get the world coordinates for the pixel, you need to do a readpixels and get that pixel's actual z-buffer value, and pass that into UnProject.

That help?

-d
0

LVL 17

Expert Comment

ID: 12018602
Sorry, clicked submit and didn't mean to...

That was from http://www.opengl.org/resources/faq/technical/glu.htm

Also look at http://www.gamedev.net/community/forums/topic.asp?topic_id=268798 for another discussion of the topic.

-d
0

LVL 1

Author Comment

ID: 12021491
I had found the description about how window Z is confusing and added a glReadPixels call to the application to get the depth value. The depth is coming back zero which causes the UnProject to generate a -1 for Z (if I understand the above). So does this mean that the Z value will always come back either zNear or zFar? Does the application need to use the depth value to scale between zNear and zFar?
0

LVL 17

Accepted Solution

davebytes earned 750 total points
ID: 12030378
If the pixel has nothing in it, it'll be whatever Z you ClearDepth to, and that should translate approximately to zFar.   Thought I'm not sure that things on the near or far plane have well defined values, may be clamped, which could easily be the -1 you are getting.

If the pixel has something in it, it should translate to the approximate z value.

I don't think you need to be doing any further scaling...

-d
0

## Featured Post

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

Recently, in one of the tech-blogs I usually read, I saw a post about the best-selling video games through history. The first place in the list is for the classic, extremely addictive Tetris. Well, a long time ago, in a galaxy far far away, I was…
Performance in games development is paramount: every microsecond counts to be able to do everything in less than 33ms (aiming at 16ms). C# foreach statement is one of the worst performance killers, and here I explain why.
This tutorial will teach you the special effect of super speed similar to the fictional character Wally West aka "The Flash" After Shake : http://www.videocopilot.net/presets/after_shake/ All lightning effects with instructions : http://www.mediaf…
We’ve all felt that sense of false security before—locking down external access to a database or component and feeling like we’ve done all we need to do to secure company data. But that feeling is fleeting. Attacks these days can happen in many w…
###### Suggested Courses
Course of the Month7 days, 13 hours left to enroll