Operating system and graphics card
Fedora 27 / GeForce GT 740
2.79a - offical release - 8928d99270f
Short description of error
Unlike the “Mesh Deform”-modifier, which is applying the changes in a relative fashion, the “Surface Deform”-modifier just overwrites the previous modifier/s with the absolute values of the transform.
Exact steps for others to reproduce the error
Please look into the attached blend-file to follow the examples:
Example1: adding two times the same “Surface Deform”-modifier on top of each other yields the same result as just one or in fact any number of copies of this modifier, thus rendering the nature of the modifier-stack here somewhat useless. In contrast e.g. the “Mesh Deform”-modifier would apply each copy of itself as a relative transform, thus building upon the previous modifiers in stack and changing the output-mesh with every copy of itself.
Example2: Using the “Multi Modifier”-option of the “Armature”-modifier usually takes the same data as the previous modifier as input. The results of the previous modifier and the “Armature”-modifiers are then mixed together, using the weights of the Vertex Group as “mixing guides”. But in the case of the “Surface Deform”-modifier being the previous modifier this seems not to apply and the “Multi Modifier”-option takes no effect at all.
Example3: Adding the “Surface Deform”-modifier on top of “Armature”-modifier overwrites the effect of the “Armature”-modifier (and all other modifiers prior to the “Surface Deform”-modifier.
( An additional oddity seems to me that there is no option to control the overall influence of the “Surface Deform”-modifier with a vertex group, as is usually the case with all other deform modifiers. This though is not part of the bug report and rather a small feature request – but if someone is already looking into the above issue, I figured it wouldn't hurt to mention it here as it is probably just a matter of copy and paste from any modifier that already has this implemented - I might be wrong here though of course - please forgive my ignorance.)
Thank you for taking your time to look into that issue.