# Anyone can tell me what s means and when the s is used in cameral model **URL:** \ **Category:** Uncategorized\ **Tags:** calib3d, imgproc\ **Created:** [March 1, 2023, 7:50am UTC](https://forum.opencv.org/t/anyone-can-tell-me-what-s-means-and-when-the-s-is-used-in-cameral-model/12193 "2023-03-01T07:50:39Z")\ **Posts on this page:** 3\ **Page:** 1
**Author:** ![Mary](https://avatars.discourse-cdn.com/v4/letter/m/e99b99/32.png) [@Mary](https://forum.opencv.org/u/Mary)\ **Post date:** [March 1, 2023, 7:50am UTC](https://forum.opencv.org/t/anyone-can-tell-me-what-s-means-and-when-the-s-is-used-in-cameral-model/12193/1 "2023-03-01T07:50:39Z")
As “The distortion-free projective transformation given by a pinhole camera model is shown below. sp=A[R|t]Pw, where … and s is the projective transformation’s arbitrary scaling and not part of the camera model.” written on opencv tutorials I am confused about which step s is used in? That is, which transformation does the projective transformation happen in? ---
**Author:** ![crackwitz](https://sea2.discourse-cdn.com/flex020/user_avatar/forum.opencv.org/crackwitz/32/14_2.png) [@crackwitz](https://forum.opencv.org/u/crackwitz)\ **Post date:** [March 1, 2023, 3:38pm UTC](https://forum.opencv.org/t/anyone-can-tell-me-what-s-means-and-when-the-s-is-used-in-cameral-model/12193/2 "2023-03-01T15:38:15Z")
it is _discarded_, not used. if you have a 2D point (x,y), then in the corresponding projective space (3D), this is represented by (x,y,1), but also all (x,y,1) \cdot s for any nonzero s you encounter this after matrix multiplication. you normalize (“dehomogenize”) the result by dividing by the last coordinate. (x,y,1) = (x s, y s, s) / s ---
**Author:** ![Mary](https://avatars.discourse-cdn.com/v4/letter/m/e99b99/32.png) [@Mary](https://forum.opencv.org/u/Mary)\ **Post date:** [March 2, 2023, 10:15am UTC](https://forum.opencv.org/t/anyone-can-tell-me-what-s-means-and-when-the-s-is-used-in-cameral-model/12193/3 "2023-03-02T10:15:30Z")
I get it. Thank you very much.