Sometimes you want to trace images that aren’t a nicely contrasted black and white. They might be line art with faint lines, colored clip art, or have several shades of gray. You can always use your favorite photo editing software to manipulate the image, but many times you can get a good trace in Inkscape by changing the “Brightness cutoff” threshold. Here is how you do that.
Images with empty or incomplete traces
Here I have a line art image with pale lines.
If I tried to trace it with the default threshold of 0.45 the preview and the trace would be empty. The Brightness cutoff threshold needs to be increased to trace the pale lines. In the image below I have increased the threshold to 0.90 since the lines are so faint. With the image selected, I have clicked on “Update” to get an idea of how the image will trace.
Since I know that traces are almost always darker than the preview, I will click “OK” to trace the image. The image below shows what the trace looked like. All of the lines are traced with no breaks or holes in them. If you had an image that traced but left gaps or small holes inside the lines, you would need to increase the threshold until the lines trace as solid lines.
Colored images, or images with dark traces
Sometimes you want to trace an image with shades of gray or with colors, such as a colored clip art, but the trace using the default settings gives a blotchy trace, or areas that are solid that shouldn’t be. Below is a colored clip art that has black sections when traced with the default settings.
The Brightness cutoff threshold needs to be decreased to remove the darker colors from the trace. In the image below I have decreased the threshold to 0.20. With the image selected, I have clicked on “Update” to get an idea of how the image will trace.
Since the preview looks good, I will click “OK” to trace the image. The image below shows what the trace looked like. If more of the image traces than was wanted, simply keep reducing the threshold and click “update” until just the black lines are traced.
If a trace is not tracing enough of the image, try increasing the Brightness cutoff threshold.
If a trace is tracing too much, try decreasing the Brightness cutoff threshold.
Some images just won’t trace well and will need to be cleaned up after tracing, or a different method should be used, such as manually outlining using the Bezier tool.