I imported a jpeg image and then used Path>Trace Bitmap to dissect it into various objects, which I modified. But when reassembling them, it would have been helpful if I could have `cropped' just what I wanted; i.e. cut out sections I didn't want. I was unable to figure out how to do that.

Any suggestions?

after tracing do you really have multiple objects or do you have a single path that may be discontinuous? if the former you may want to group the objects then draw a path that outlines the parts you'd like to keep and do object>clip. if it's a single path you can skip the grouping and do the same thing.

I'm not sure just what I have. When I did Path>Trace Bitmaps, I used Colors. I got something which I then applied Ungroup to. The result was two separate collection of bounded regions, each of which was repeated in different colors. I deleted the extra copies, but I was unable to further decompose any of the remaining objects. So I went back to the photo and cropped to make separate images, each containing a piece of what I wanted. I then applied the above process to each of these separately.

So I ended up with bounded, nonconvex regions, but I don't know what Inkscape thought they were. I was able to modify them as desired by changing the fill color. Finally I reassembled them by importing them in a single document, using shift click to collect everything I wanted together, and grouping the result as a single object. Unfortunately some of the sub-objects had extra whitespace surronding the desired region. I couldn't figure out how to crop them further within Inkscape, so I saved the result as a bitmap (.png file), and used gimp to crop it as desired. But my Inkscape original (saved as an .svg file) still remains uncropped.

Let me try to be more specific about the crux of the problem.

Path>Trace Bitmap produces a collection of objects which can be ungrouped into separate objects. In my case I got closed polygonal regions withiin a larger rectangle of whitespace. I couldn't see any way to select just the closed polygonal region without the bounding rectangle.

I was trying to modify the sizes of the bouding rectangles so they were consistent with one another. I still want to know how to do that, but if I could figure out how to pull out the polygonal region as a separate object, that would be better

the color scan produces one path for each level of color. when you ungroup and then select the object, look at the bottom info bar and it should tell you what type of object you have. it will almost certainly say it is a path. you can then bound the piece you want with another path or rectangle or ellipse and do object>clip, or you can use the node editor tool to delete the portions of the path you don't want. beware the second option, however as the paths formed by the trace function don't always flow the way you think they do.