Update params.md

doc/params.md

@@ -24,7 +24,7 @@ In the table below, the various possible parameters are listed. If nothing is li
 | thr_ratio  | float      | 1.25          | (0,inf)     | Only if you need to | Adjusts the adaptive thresholding of the input image. Values > 1 will favour foreground regions (more white in thresholded image) and values < 1 will favour background regions (more black in thresholded image). |
 | thr_win_pc | float      | 0.2           | \[0,1]      | Only if you need to | Adjusts the size of the neighbourhood window to use for adaptive thresholding of the input image, specified as a percentage of the width of the tracking window. Larger values avoid over-segmentation, whilst smaller values make segmentation more robust to illumination gradients on the trackball. |
 | vid_codec  | string     | h264          | [h264,xvid,mpg4,mjpg,raw] | Only if you need to | Specifies the video codec to use when writing output videos (see `save_raw` and `save_debug`). |
-| sphere_map_fn | string  |               |             | Only if you need to | If specified, FicTrac will attempt to load a previously generated sphere surface map from this filename. |
+| sphere_map_fn | string  |               |             | Only if you need to | If specified, FicTrac will attempt to load a previously generated sphere surface map from this filename. Note that if you set this option, you should probably also set `opt_do_global` otherwise FicTrac may not find the initial sphere attitude. |
 |            |            |               |             |                     |             |
 | opt_max_evals | int     | 50            | (0,inf)     | Probably not        | Specifies the maximum number of minimisation iterations to perform each frame. Smaller values may improve tracking frame rate at the risk of finding sub-optimal matches. Number of optimisation iterations is printed to screen during tracking (its=...). |
 | opt_bound  | float      | 0.35          | (0,inf)     | Probably not        | Specifies the optimisation search range in radians. Larger values will facilitate more track ball rotation per frame, but result in slower tracking and also possibly lead to false matches. |