Creating one object from a shape and paths in Inkscape

Question

Following up from an earlier question, I am tracing a street map for a town using a bitmap from OpenStreetMap. I am using bezier curves for the roads. Different roads have different colors and sizes, shown in the figure by the green and yellow lines. I have also created a bounding box for the map, shown in graphic as a rectangle with a black stroke and a gray fill.

There is also a river crossed by a road bridge at the top of the map - only part of the river is shown. The shoreline is drawn in blue. I want to shade the river between the shorelines in light blue. I have realised that I can use the bucket tool to fill that area, but that feels like the wrong approach. Given that the area of water is bounded by the shorelines and the bounding box, I had expected to be able to use that to create an object.

Thinking that the fact that the boundary box is a shape might be the issue, I also tried adding paths to use to combine with the shorelines into one closed path, like this (new lines in red):

This didn't work as I could not fill the resulting shape:

Any suggestions as to how I could create an object from the bounding box and blue shorelines?

Answer 1

You need to make a closed path to make an object properly fillable. This is basic concept in all vector image editing software, not just Inkscape. I'm assuming you don't really know what that means, so I'll try to explain.

In the example below, the shape on the left is made of independent lines. They are not one object, but actually four separate objects. When you try to apply a fill, it will add the fill to each object separately.

In the example on the right, this is one shape made with the Bézier tool. It's just one closed continuous path, which is fillable.

The easiest way to do this is to draw a closed path, using the Bézier tool, and create the object in one go. When you close the path, you should see the last (and first) node highlight red. Like this:

It is possible to join separate lines in Inkscape, but it's a messy process. Basically you have to select two end nodes and use the Join button. More trouble than it's worth.

If you need the image to follow the eges of the grey rectangle you could use enable Snapping, Snapping to Nodes, and Snap to Paths.

Example