[edit] I withdraw my comment.
[This message has been edited by knackered (edited 03-07-2002).]
knackered,
Although you are not very polite, I still would like to thank you for reminding me that I can go back to the original point and solve the problem in a mathematic way. Before I was thinking only how to get the infromation from OpenGL functions.
You said we can find the intersection of two planes in order to solve the problem in one of your previous post, however, that requries you to know the equations of the planes and we do not have these equations at hand. I would rather use coordinate transformation to do that since our users could rotate the image frequently.
I will also take and try marcus256, Rob The Bloke, zeckensack and other people’s advice because I want to learn something. knackered, do you mean it is very hard to do so in you last post?
Thank you, marcus256, This is a beautiful way! I learn a lot from you!
Originally posted by marcus256:
[b]I do not understand why people are so afraid of stencil buffers. It’s an excellent way to store “invisible” information about rendered geometry.
I made a post that hopefully solves the problem gracefully: http://www.opengl.org/discussion_boards/ubb/Forum2/HTML/007650.html
I hope it is correct (haven’t tried it).[/b]
This is the most hilarious thread I have ever read. How many times can you tell a man a dollar is a dollar before he understands. I know you have smiley faces 
but don’t you have a belly laugh, where the icon laughs so hard it rolls around, kicks up dust, and sticks its feet in the air.
But seriously Mr. Yao. If you can’t understand all these responses no one can help you.
Concerning of your opinion on cross-posting and double-posting, it is generally considered inappropiate in this forum. Don’t ask me who and why set the stupid rules. Posting things multiple times only would piss certain people. It will also reduce the chance to get suggestions from these people. Since you know all real specialists are people of patience, you probably don’t concern their opinions anyway.
I learned how to get the equation of a plane from three points when I was in grade 9, just like my classmates. It is just computationally cheaper to solve it geometrically.
Furrage, I’m glad someone finds this funny - I was beginning to think I was going mad
Mr. Yao, I’m so glad you finally understand. You have a low opinion of me now, but I’m not in a popularity contest, to coin a phrase
I can think of some good reasons to dislike multiple posts.
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Imagine someone entering 10 posts in the forum on the same topic. That means 9 other posts get moved to the second page where they’re unlikely to be seen. Thats 9 other persons who will never get an answer because someone is inconsiderate.
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Imagine someone puts 10 posts under 10 different topic names even though they’re the same. That’s 9 topics a reader has to go through unnecessarily wasting time. That time could be spent dealing with another post. Heck, even if it’s the same name most people still end up wasting time wading through multiple posts.
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People hate repeating answers. It’s caught me already where I answered one post, came across a similar one, was in the process of posting to the second which included a link to the first answer, when I discovered the post was from the same person. That was a real waste of time.
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Telephone companies and some ISPs love this time wasting. Especially when people use 56k or less modems.
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As stated above, multiple posts tick people off, so they’ll boycott your post and you won’t get the full range of expert feedback you’re looking for.
So please be considerate and stick to one post in one forum. If the chosen forum can’t help try another, but give your first choice a chance before moving on. Also use the search facility. A lot (and I do mean a lot) of your questions have already been answered and are archived in this forum.
And if nothing else read the current posts before making your own. Yesterday, there were three or more current posts on picking from different persons asking the same question. If two persons had just taken the time to read first there would be no need for the additional posts.
Woo, no technical reply after a weekend’s rest! 
Since I do not visit this forum frequently, I do not know some tacit rules such as no cross-posting and double-posting. When I first made the post, I just wanted to increase the opportunities of getting quick answers. I did not intend to waste your time. I apologize for the inconvenience I brought to you all and I promise that I will not do it next time.
I am not an unreasonable person. If someone reminds me the rules politely, I will be happy to accept their suggestions. However, someone just wants to show how smart he is by insulting others.
I admit that I was stupid because when I encountered the problem, I thought only of asking help from OpenGL, I did not even thinking about solving the problem in Geometry. I guess you understand what I mean when I say this.
There do exist some people who do not waste time in blaming people, but offer elegant solutions and get fun from helping people. I appreciate their help. I somewhat agree with angryGLfan, make beginners feel easy!