| Drawing 3D Objects properly [message #31975] |
Thu, 05 September 2002 00:49  |
MKatz843
Messages: 98
Registered: March 2002
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Member |
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I'm very comfortable with object graphics in 2D. Now I've started
branching out into the 3rd dimension. (No, I haven't yet seen the new
object graphics book.)
Whatever I try, I can't seem to get solid-fill polygons to
successfully "hide" or "block" another from view. With texture-mapped
polygons it works. I've tried futzing with REJECT = [0,1,2], BOTTOM
colors, you name it.
I'm just drawing 6-sided, rectangular, 3D boxes. By changing the
handedness of the vectors I can make the surface seem to disappear in
an undesirable way, but I can't get one surface to block another.
Any tips? This problem should be too simple for IDLgrVolume objects.
I'd like to stick to IDLgrPolygon objects, if possible.
Thanks,
M. Katz
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| Re: Drawing 3D Objects properly [message #32039 is a reply to message #31975] |
Fri, 06 September 2002 19:29  |
David Fanning
Messages: 11724
Registered: August 2001
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Senior Member |
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Rick Towler (rtowler@u.washington.edu) writes:
> Sure as sure can be. I just looked at the evidence again. The top and
> bottom windows look identical.
>
> Last I knew you had a nVidia Quadro2 based video card. I have an nVidia
> card too so that is a bit odd that you would see something different. Maybe
> nVidia has been doing some work with their z-buffer. I am running the 30.82
> drivers with a GF3.
Ah, I was running with the software renderer, not the
hardware. With hardware, everything renders correctly.
(Which, of course, is just the opposite of what usually
happens.) The ZCLIP tip offered by Karl also clears
up the problem for both hardware and software rendering.
Cheers,
David
--
David W. Fanning, Ph.D.
Fanning Software Consulting, Inc.
Phone: 970-221-0438, E-mail: david@dfanning.com
Coyote's Guide to IDL Programming: http://www.dfanning.com/
Toll-Free IDL Book Orders: 1-888-461-0155
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| Re: Drawing 3D Objects properly [message #32041 is a reply to message #31975] |
Fri, 06 September 2002 14:56 |
Rick Towler
Messages: 821
Registered: August 1998
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Senior Member |
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"David Fanning" <david@dfanning.com> wrote ...
> Rick Towler (rtowler@u.washington.edu) writes:
>
>> Your problem must be a Mac thing? It looks good to me with either
>> projection. Attached is an image showing your programs output.
>> Nix that image, my news server won't let me post it (sheesh, it is
>> only 8k). But what I did see looked correct. I can send the
>> image off list if you wish.
>>
>> {x86 Win32 Windows Microsoft Windows 5.5 Aug 28 2001 32 64}
>
> Humm. That's odd. It looks wrong to me in both IDL 5.4
> and IDL 5.5 on my Windows machine. Are you *sure* it looks
> correct. In the second window, the red plane never goes
> behind the green plane.
Sure as sure can be. I just looked at the evidence again. The top and
bottom windows look identical.
Last I knew you had a nVidia Quadro2 based video card. I have an nVidia
card too so that is a bit odd that you would see something different. Maybe
nVidia has been doing some work with their z-buffer. I am running the 30.82
drivers with a GF3.
-Rick
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| Re: Drawing 3D Objects properly [message #32051 is a reply to message #31975] |
Thu, 05 September 2002 18:29 |
David Fanning
Messages: 11724
Registered: August 2001
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Senior Member |
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Rick Towler (rtowler@u.washington.edu) writes:
> Your problem must be a Mac thing? It looks good to me with either
> projection. Attached is an image showing your programs output.
> Nix that image, my news server won't let me post it (sheesh, it is
> only 8k). But what I did see looked correct. I can send the
> image off list if you wish.
>
> {x86 Win32 Windows Microsoft Windows 5.5 Aug 28 2001 32 64}
Humm. That's odd. It looks wrong to me in both IDL 5.4
and IDL 5.5 on my Windows machine. Are you *sure* it looks
correct. In the second window, the red plane never goes
behind the green plane.
Cheers,
David
--
David W. Fanning, Ph.D.
Fanning Software Consulting, Inc.
Phone: 970-221-0438, E-mail: david@dfanning.com
Coyote's Guide to IDL Programming: http://www.dfanning.com/
Toll-Free IDL Book Orders: 1-888-461-0155
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| Re: Drawing 3D Objects properly [message #32054 is a reply to message #31975] |
Thu, 05 September 2002 14:07 |
Karl Schultz
Messages: 341
Registered: October 1999
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Senior Member |
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Try reducing the ZCLIP range..
If I made ZCLIP=[999,-999] / 100, the problem went away.
Here's why:
The hardware Z buffer has limited precision. Many Z buffers have only 16
bits per pixel and it varies with the implementation. This may explain why
the problem appears worse on some systems as compared to others. In
addition, if you are on a machine that supports OpenGL, you may see some
differences between the hardware and software renderers, because the Z
buffer depths may be different between the OpenGL on your machine and the
IDL software renderer (Mesa).
When you draw a unit polygon like this that takes up about 2 units in a Z
buffer range of [-999, 999], you are only using 1/1000th of the Z buffer.
So, if the Z buffer is 16-bit, you are only going to be using 64 of the
possible 64K Z buffer values. This results in essentially 64 discreet
"depth" values used while drawing these two polygons. If the polygons get
close enough in Z as you spin them around, the Z values of some of the
pixels after the view projection will map to the same Z buffer value. When
there is a tie in Z, the drawing order becomes the deciding factor.
The graphics system (OpenGL) maps the Z part of the viewport to the Z buffer
differently, depending on the PROJECTION, which may account for the
difference there. When the projection is perspective, the ZCLIP-to-Zbuffer
mapping is not linear. This means that you might be able to resolve the
polygons at one end of the ZCLIP space and not the other. This explains why
you were not able to resolve the polygons in the perspective projection.
If you were to make the ZCLIP values really big, you'd start to see
stitching effects in both projections.
In general, you don't want to set your ZCLIP values to such large numbers.
It is better to get them closer to the actual extents of your model.
This problem did not show up in XOBJVIEW because XOBJVIEW walks the graphics
tree you provide to determine the Z extents of your data.
Try setting ZCLIP to [10,-10].
Hope this helps,
Karl
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| Re: Drawing 3D Objects properly [message #32057 is a reply to message #31975] |
Thu, 05 September 2002 11:33 |
Rick Towler
Messages: 821
Registered: August 1998
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Senior Member |
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Yes, xobjview is a *very* useful tool. Too bad it doesn't get any ink.
Your problem must be a Mac thing? It looks good to me with either
projection. Attached is an image showing your programs output.
Nix that image, my news server won't let me post it (sheesh, it is
only 8k). But what I did see looked correct. I can send the
image off list if you wish.
{x86 Win32 Windows Microsoft Windows 5.5 Aug 28 2001 32 64}
-Rick
"M. Katz" <MKatz843@onebox.com> wrote in message
news:4a097d6a.0209051008.6857593@posting.google.com...
> Thanks for the tip to use xobjview! In xobjview, I saw that the 3D
> objects behaved as they are supposed to. This led me to tweak all of
> the settings on my IDLgrView, until I found the culprit.
>
> The PROJECTION keyword is supposed to change from orthogonal to a
> near/far view. In IDL 5.4 (at least), I see that it does more than
> that. If you run my simple side-by-side test script below, you'll see
> the issue.
>
> With PROJECTION=1, one polygon successfully hides the one behind it.
> With PROJECTION=2, the red square (first drawn) *always* blocks the
> green square.
>
> Run the test script below by copying it into a new file, name and save
> the file, and then .run the file. Move the mouse around in the IDL 0
> window (the third window on screen, possibly on the right of the other
> two). You can control the rotation direction and velocity. Click in
> that window to quit.
>
> I'm very interested to know if this is platform speciic or not. (I'm
> running IDL in Classic mode on MacOS 10.1.5)
>
> M. Katz
> (Script Follows)
>
> ;--------------------------------------------
>
> ; object-viewer test script demonstrating projection diffrerences
>
> ;--- create two identical windows
> mywindow1 = obj_new('IDLgrWindow', quality=2, dimensions=[200,200], $
> LOCATION=[50,50])
> mywindow2 = obj_new('IDLgrWindow', quality=2, dimensions=[200,200], $
> LOCATION=[50,300])
> ;--- create two views with different PROJECTION values
> view1 = obj_new('IDLgrView',EYE=1000, zclip=[999,-999], $
> color=[0,0,0], VIEWPLANE_RECT = [-1,-1,2,2]*2, PROJECTION=1)
> view2 = obj_new('IDLgrView',EYE=1000, zclip=[999,-999], $
> color=[0,0,0], VIEWPLANE_RECT = [-1,-1,2,2]*2, PROJECTION=2)
>
> ;--- Create two identical models: A red and a green square with
> different z values
> model1 = obj_new('IDLgrModel')
> model1 -> Add, obj_new('IDLgrPolygon', [-1,1,1,-1], [1,1,-1,-1], $
> [0,0,0,0]-0.5, COLOR=[200,50,50])
> model1 -> Add, obj_new('IDLgrPolygon', [-1,1,1,-1], [1,1,-1,-1], $
> [0,0,0,0]+0.5, COLOR=[50,200,50])
>
> model2 = obj_new('IDLgrModel')
> model2 -> Add, obj_new('IDLgrPolygon', [-1,1,1,-1], [1,1,-1,-1], $
> [0,0,0,0]-0.5, COLOR=[200,50,50])
> model2 -> Add, obj_new('IDLgrPolygon', [-1,1,1,-1], [1,1,-1,-1], $
> [0,0,0,0]+0.5, COLOR=[50,200,50])
>
> view1 -> Add, model1
> view2 -> Add, model2
>
> ;--- Allow the user to manipulate the graphics as they are displayed
> window, xsize=200, ysize=200
>
> repeat begin
> cursor, xc, yc, /normal, /nowait
>
> model1 -> Rotate, [0,1,0], (xc-0.5)*5
> model1 -> Rotate, [1,0,0], (yc-0.5)*5
> model2 -> Rotate, [0,1,0], (xc-0.5)*5
> model2 -> Rotate, [1,0,0], (yc-0.5)*5
>
> mywindow1 -> Draw, view1
> mywindow2 -> Draw, view2
> endrep until (!mouse.button GT 0)
>
> end
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| Re: Drawing 3D Objects properly [message #32059 is a reply to message #31975] |
Thu, 05 September 2002 11:08 |
MKatz843
Messages: 98
Registered: March 2002
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Member |
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Thanks for the tip to use xobjview! In xobjview, I saw that the 3D
objects behaved as they are supposed to. This led me to tweak all of
the settings on my IDLgrView, until I found the culprit.
The PROJECTION keyword is supposed to change from orthogonal to a
near/far view. In IDL 5.4 (at least), I see that it does more than
that. If you run my simple side-by-side test script below, you'll see
the issue.
With PROJECTION=1, one polygon successfully hides the one behind it.
With PROJECTION=2, the red square (first drawn) *always* blocks the
green square.
Run the test script below by copying it into a new file, name and save
the file, and then .run the file. Move the mouse around in the IDL 0
window (the third window on screen, possibly on the right of the other
two). You can control the rotation direction and velocity. Click in
that window to quit.
I'm very interested to know if this is platform speciic or not. (I'm
running IDL in Classic mode on MacOS 10.1.5)
M. Katz
(Script Follows)
;--------------------------------------------
; object-viewer test script demonstrating projection diffrerences
;--- create two identical windows
mywindow1 = obj_new('IDLgrWindow', quality=2, dimensions=[200,200], $
LOCATION=[50,50])
mywindow2 = obj_new('IDLgrWindow', quality=2, dimensions=[200,200], $
LOCATION=[50,300])
;--- create two views with different PROJECTION values
view1 = obj_new('IDLgrView',EYE=1000, zclip=[999,-999], $
color=[0,0,0], VIEWPLANE_RECT = [-1,-1,2,2]*2, PROJECTION=1)
view2 = obj_new('IDLgrView',EYE=1000, zclip=[999,-999], $
color=[0,0,0], VIEWPLANE_RECT = [-1,-1,2,2]*2, PROJECTION=2)
;--- Create two identical models: A red and a green square with
different z values
model1 = obj_new('IDLgrModel')
model1 -> Add, obj_new('IDLgrPolygon', [-1,1,1,-1], [1,1,-1,-1], $
[0,0,0,0]-0.5, COLOR=[200,50,50])
model1 -> Add, obj_new('IDLgrPolygon', [-1,1,1,-1], [1,1,-1,-1], $
[0,0,0,0]+0.5, COLOR=[50,200,50])
model2 = obj_new('IDLgrModel')
model2 -> Add, obj_new('IDLgrPolygon', [-1,1,1,-1], [1,1,-1,-1], $
[0,0,0,0]-0.5, COLOR=[200,50,50])
model2 -> Add, obj_new('IDLgrPolygon', [-1,1,1,-1], [1,1,-1,-1], $
[0,0,0,0]+0.5, COLOR=[50,200,50])
view1 -> Add, model1
view2 -> Add, model2
;--- Allow the user to manipulate the graphics as they are displayed
window, xsize=200, ysize=200
repeat begin
cursor, xc, yc, /normal, /nowait
model1 -> Rotate, [0,1,0], (xc-0.5)*5
model1 -> Rotate, [1,0,0], (yc-0.5)*5
model2 -> Rotate, [0,1,0], (xc-0.5)*5
model2 -> Rotate, [1,0,0], (yc-0.5)*5
mywindow1 -> Draw, view1
mywindow2 -> Draw, view2
endrep until (!mouse.button GT 0)
end
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| Re: Drawing 3D Objects properly [message #32066 is a reply to message #31975] |
Thu, 05 September 2002 08:51 |
Rick Towler
Messages: 821
Registered: August 1998
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Senior Member |
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Can you post a simple example using xobjview to display?
-Rick
"M. Katz" <MKatz843@onebox.com> wrote in message
news:4a097d6a.0209042349.663aca19@posting.google.com...
> I'm very comfortable with object graphics in 2D. Now I've started
> branching out into the 3rd dimension. (No, I haven't yet seen the new
> object graphics book.)
>
> Whatever I try, I can't seem to get solid-fill polygons to
> successfully "hide" or "block" another from view. With texture-mapped
> polygons it works. I've tried futzing with REJECT = [0,1,2], BOTTOM
> colors, you name it.
>
> I'm just drawing 6-sided, rectangular, 3D boxes. By changing the
> handedness of the vectors I can make the surface seem to disappear in
> an undesirable way, but I can't get one surface to block another.
>
> Any tips? This problem should be too simple for IDLgrVolume objects.
> I'd like to stick to IDLgrPolygon objects, if possible.
>
> Thanks,
>
> M. Katz
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