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glMultMatrixf help needed

Started by greentea8202 Jul 4, 2004 at 9:53 PM 3 replies 600+ views

Original Post

Jul 4, 2004 at 9:53 PM

Hi, I'm using arcball for my rotation control and I generated 4x4 matrix for rotation. However, if I try to use same approach for translation, I somehow get weird rendering problem. I get same modelview matrix I get by using glTranslatef, for using glMultMatrixf, but my object just shakes whenever I try to translate it. For example, glTranslatef(2.0, 2.0, 0.0) is equal to glMultMatrix(m) where m = 1 0 0 0 0 1 0 0 0 0 1 0 2 2 0 1. And current modelview matrix would be same as matrix m since starting of matrix of display() is identity matrix. Why does image shakes rather than moving smoothly as gltranslate function? Please help me out~! Thanks!!!

vincoof

Jul 5, 2004 at 9:03 AM

Shaking is certainly due to floating-point precision errors. Try using glMultMatrixd instead and this should be better.
As for why glTranslatef works correctly (ie doesn't need glTranslated to get good precision), that I do not know. Maybe that's some kind of driver optimization.

mikeman

Jul 5, 2004 at 9:11 AM

Quote:
Original post by vincoof
Shaking is certainly due to floating-point precision errors. Try using glMultMatrixd instead and this should be better.
As for why glTranslatef works correctly (ie doesn't need glTranslated to get good precision), that I do not know. Maybe that's some kind of driver optimization.

Bah,I don't think precision issues would result in artifacts so obvious that make the object shaking.It should be something with the order of columns and rows.Try using
m=1 0 0 2
0 1 0 2
0 0 1 0
0 0 0 1

vincoof

Jul 5, 2004 at 2:15 PM

At first glance, I also thought it was row-major vs column-major matrix issue. But I don't think it would ever shake if the matrix was transposed. Rather it would look very weird, stretched along the XY axis

greentea8202

Jul 6, 2004 at 1:55 PM

thanks guys for your reply.
I sort of figured out what I did .. made some stupid mistake~ :p

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