From background masking option usually applicable only for turntable scenarios when camera position is fixed, background is static and only object is rotating; or for fixed camera rigs, which take images simultaneously. Idea of such method is that captured images of the background (that wouldn't change during the image acquisition process) and use such "empty" images for the masks generation.
Please note that for each camera placement you will need to use own mask. For example if you use two capturing positions, each of them will require it's own "empty" background image.
Generally your workflow should be:
1. Add photos using Add Photos... command from the Workflow menu.
2. In the Add Photos dialog browse the source folder and select files to be processed. The images will be added to the active chunk of the project.
3. Select Import masks from context menu of the Photos pane after right-clicking on selected images in the pane.
After that mask will be applied automatically and cut off all parts of the background that software can find.
- Parameters Tolerance is considered for the "from Background" method only. It specifies the tolerance threshold used for background differencing. Tolerance value should be set according to the color separation between foreground and background pixels. For larger separation higher tolerance values can be used.
- A mask can be inverted using Invert Mask command from the Edit menu. The command is active in Photo view only. Alternatively, you can invert masks either for selected cameras or for all cameras in a chunk using Invert Masks... command from a photo context menu on the Photos pane.
- You can edit the mask manually if necessary, check a detailed article about manual mask drawing here: - Masks.
4. Select Align photos... command from the Workflow menu.
You can apply masks to Key/Tie points:
- If Apply masks to Key point selected - masked areas are excluded from feature detection procedure independently for each photo.
- If Apply masks to Tie points selected - certain tie points are excluded from alignment procedure. Effectively this implies that if some area is masked at least on a single photo, relevant key point on the rest of the photos picturing the same area will be also ignore during alignment procedure (a tie point is a set of key point which have been matched as projections of the same 3D point on different images). This can be useful to suppress background in turntable - like shooting scenario with few or even with ta single mask.
- If you run Workflow > Align Photos... without masks (or with Apply masks to None, which effectively is the same option), you will find cameras incorrectly aligned with respect to the background. All camera positions will be estimated as same, since camera, in fact, was at the same place on a tripod.
- Note that photo of the background (without object) is not mandatory. You can mask background on one of the photos observing the object and all cameras will be also aligned. Just don't forget to use Apply masks to Tie points feature and try to mask background on the photo the observes as much of the background as possible. Sometimes you will need to mask more than one photo, because in some cases there is no single photo observing the whole background surface at once.