The Marking Residual Display setting on the Visibility on Photos pane is used to control the Marking Residual display on Photographs.
When residual errors are displayed each Photograph will show a black bar starting at the Marked Point and going off in some direction. Where the black bar ends is where PhotoModeler thinks the Marked Point should be given other data such as referenced points, Camera Station orientations, Camera information, and any Control Point locations or Constraints.
In a well solved project, these Marking Residuals (also called residual errors) should all be very small. In fact they may be so small that you cannot see them when the Marking Residual Display comes up. Try using the zoom tool and zoom in on a Marked Point to see if the black bar is indeed there.
To get an overview of the errors when they are all small, there is a way to set exaggeration factor (see Visibility on Photos). When this factor is set to 1 (the default) then the end of the bar is where PhotoModeler thinks the point should be, when this factor is greater than 1, the bars are magnified by the factor. For example, if the magnification factor is 10 then the black bars are all drawn 10 times longer than reality but they still point in the same direction.
The Residual Marking Display can be very useful in tracking down certain types of marking and referencing errors.
a) Marking errors: if you have a point that has 5 references (appears on 5 photographs) but has been incorrectly placed on one of those photos then the Marking Residual error for the point will be large and will point in the direction on the photograph where it should be marked. This display will not work well though when there are not many references (i.e. 2 or 3 references) and one of the points is marked incorrectly. In this case, all the references on all the photographs will appear to have larger Residual Marking Errors.
b) Referencing errors: if you have misreferenced a point (for example referenced the right headlight point on photo 1 to the left headlight on photo 2), the Marking Residuals for these points will be high and will point at the correct point to be referenced.
If you use a large magnification factor (over 15) do not be surprised by some large Marking Residuals. Some of these might be real problems that you should investigate but otherwise it is just showing the uncertainty in the system. Very high quality projects and some calibrations may require magnifications in the hundreds before the residuals and their patterns are visible.
Note: The Projection display is very similar to Marking Residuals. If you turn on 3D Points display in the Visibility on Photos pane and then turn on Marking Residual display with a magnification of 1.0 (which means "no exaggeration"), you will see that the residual vector is drawn between the marked point and the projected point. This is a good illustration of what a Marking Residual is.