RasterTracer is a plugin for semi-automatic digitizing of an underlying raster layer in QGis.
It is useful, for example, when you need to digitize a scanned topographic map, with curved black lines representing lines of equal heights of the surface (contours). Instead of creating this curved vector line by manually clicking at each segment of this curved line, with this plugin you can click at the beginning of the curved line and at the end of the curved line, and it will automatically trace over black pixels (or pixels that are almost black) starting from the beginning to the end. By using this plugin you reduce clicks while digitizing raster maps.

The process is show here:

Tracing is enabled only if the selected vector layer is in the editing mode.

The geometry type of the vector layer has to be MultiLineString / MultiCurve.

You can choose the color that will be traced over in the raster image. To do this, check the box trace color and select the desired color in the dialog window.

If trace color is not checked, the plugin will try to trace the color that is similar to the color of the pixel on the map at the place where you clicked the last time. This means that each time you click on the map, it will trace a slightly different color. This slows down tracing a bit, but may be useful if the color of the line you are tracing varies over the map.

Right now the plugin can trace images that have a standard RGB color space. It has no support for any black and white, grey, or indexed images. This means that if your image has an unsupported colorspace, you have to convert the colorspace of your image to RGB first.

Also in the current version there are some issues when coordinate system of the raster layer differs from the coordinate system of the project.

b - delete last segment

a - switch between "trace" mode and "straight-line" mode.

Esc - cancel tracing segment. Useful when raster_tracer struggles to find a good path between clicked points (Usually when points are far from each other).