How to use free scaling parameter (alpha) when dealing with getOptimalNewCameraMatrix and stereoRectify : should one use the same value ?

As far as I understand it, I guess a few things that led me to this question are worth to be listed:

• In getOptimalNewCameraMatrix, OpenCV doc says "alpha Free scaling parameter between 0 (when all the pixels in the undistorted image are valid) and 1 (when all the source image pixels are retained in the undistorted image)" [sounds to me like 1 = retain source pixels = minimize loss]

• In stereoRectify, OpenCV doc says "alpha Free scaling parameter.... alpha=0 means that ... (no black areas after rectification). alpha=1 means that ... (no source image pixels are lost)

• So in the end alpha, seems to be a parameter that may "act" the same way ? (1 = no source pixel lost - sounds like, not sure here)

• As far as I understand it, after calibrateCamera, one may want to call getOptimalNewCameraMatrix (computing new matrices as outputs) and then stereoRectify (using new computed matrices as inputs) : do one want to use the same alpha?

Are these 2 alphas the same? Or does one want to use 2 different alphas?

CodePudding user response：

The alphas are the same.

The choice of value depends entirely on the application. Ask yourself:

Does the application need to see all the input pixels to do its job (because, for example, it must use all the "extra" FOV near the image edges, or because you know that the scene's subject that's of interest to the application may be near the edges and you can't lose even a pixel of it)?

• Yes: choose alpha=1
• No: choose a value of alpha that keeps the "interesting" portion of the image predictably inside the undistorted image.

In the latter case (again, depending on the application) you may need to compute the boundary of the undistorted image within the input one. This is just a poly-curve, that can be be approximated by a polygon to any level of accuracy you need, down to the pixel. Or you can use a mask.