Questions/Comments on Clipping
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I thought I would start this thread with the hope of discussing the fundamentals of computer based modeling, in particular as it has to do with Sketchup. I know a lot of you use more than one modeling program.
Specifically, I would like to know the inner and outer limits of the Sketchup model space. Many of you have a working or intuitive understanding of those limits. I know this is directly related to OpenGL, the display language that puts the picture to the screen.
What I have read indicates that there is a View Volume, and this relates to the "clipping problem". I think it says that the view volume is shaped like a frustum, or a pyramid with the apex cut off. The frustum is oriented along the camera view axis to simulate the range of view of the camera. The frustum is an idealized cone of vision. It is all related to the "economy of effort" in depicting information in the file. If OpenGL determines that file data is out of view, it does not display. The trick is to decide how far outside the screen to clip.
Amazingly, there seems to be little in the way of graphics/diagrams in the OpenGL pages to describe what they are talking about. Evidently these guys are not visually oriented, yet they design tools to visualize stuff. Whuh?
Is Sketchup designed with specific limitations in this area different from other programs, or do they all share the same limits?
Can someone point me to better explanations? -
SketchUp high demand on the graphics card, usually for nVIDIA better than ATI, SketchUp OpenGL for large models 3dmax excellent performance is not optimal.
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Well, ODM:
It appears I am denser in the head than most. I did not see how your reply pertained? -
ODM there is no active verb in your sentence. Therefore, you did not say anything.
mitcorb,
if you're concerned about clipping planes, this is a typical situation in which you would run into this problem: when modelling something that is 100 times as wide and long as it is high, for example a city planning model. sadly there is no way to adjust the values for the clipping planes in SU. However if you cut up the model in smaller pieces, it will not be a problem for those pieces. But when you put them back together, it might so be again.
the problem is not applicable to renderers used in SU, as they define their own clipping planes, plus they usually use the CPU, not the GPU.
Also if your level of detail is very high compared to the overall dimensions of the model, you will run into this problem when zooming in to details. The aforementioned workaround will be more succesful here.Nice mathematical description by the way.
Cheers! -
Mitcorb - I have the feeling that the description you present doesn't really reflect what's happening. As Martha notes most of us experience clipping when trying to zoom in close. The area clipped seems like it should definitely be within the field of view. If the problem relates to some view algorithm you'd think it would be fixed by now.
In a recent thread started by Hellnbak I posted what I hoped was practical advice on clipping as it seems that often clipping is related to things we may be able to avoid - like stray geometries far from the axis origin:
@unknownuser said:
The clipping plain problem has always been present in SketchUp. If you do a search here for it you'll find hours of reading. You may have noticed some models experience it more than others. Factors that can trigger it are very large models --- not large as in file size but ones that take up lots of 3d space. For instance if you try modelling an airport with all its runways you will experience clipping more than just modelling a single jet. Even with "small" models you will experience the clipping if any part of the model is far from the axis origin. It is good practice to always model close to the axis origin when you can. If you are and still have a problem it may be that there is some stray geometry (like a short line) far away that may be screwing things up. A way to check is to turn on display of hidden geometry (in the view menu) and zoom to extents. Doing that you may find there was some line or other element located far away from the rest of the model. If you eliminate it the clipping problem can improve.
Sometimes it seems no matter what you do it is near impossible to zoom in close enough to fix a particular problem. #%&@ that clipping plane! At those times you can try switching from the typical perspective view to the "parallel projection" display mode and see if it allows you to get in closer. I will do that, fix the problem, and then return to perspective mode.
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Thanks, Pyroluna and Ross:
Yes, I appreciate what you say here.
I am just trying to get my brain to perceive the extreme limits of the program in terms of the size of the virtual space. It appears to be a proportional issue. It also appears that the mathematics of display become distorted out at those extremes, but I don't know that.Or, how about this? How far to the fence- 30 meters, 300 meters 3,000 meters 30,000 meters? Likewise to the roof?
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@mitcorb said:
Well, ODM:
It appears I am denser in the head than most. I did not see how your reply pertained?Mitcorb:
Sorry, My english is poor. I can not express my idea right, I'm trying to learn English.
I mean, SketchUp model of the size of the operation, and the computer's processor, memory and video card related. If you have a good computer, especially the graphics card, another big point of your model is not a problem.
hope you can understand, thank you.ODM.
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@ODM:
Thank you for your reply. Yes, it is understood that with the right hardware there should be minimal problems with the program.
But inherent in the program(s) are limitations that cause display problems such as clipping, which can happen when geometry is created sufficiently far from 0,0,0 or when geometry is created sufficiently small enough, or when there is a significant amount of geometry. Knowing all of this from many discussions in this forum by others has not answered what the thresholds really are.Maybe I need to read some more in OpenGL pages. Perhaps the information is there and I have overlooked it.
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