Sketch and line art filters are some of the most common image effects that picture editors let you apply. While the effect is very common, there are few apps that execute it well. linesmART is a free Mac app that renders the line and sketch effect fairly well for animate objects, and exceptionally for inanimate ones. The app allows you to import any image, and applies the sketch effect to it. As opposed to turning the entire image black & white (which most apps do) this app retains the color and allows you to adjust the intensity, contrast, brightness, exposure and saturation.
The interface is simple but doesn’t support drag & drop, nor can you use it capture an image via your webcam. To import an image into the app, click Add. The app will apply the effect automatically. Click the cog wheel in the upper right corner to open the settings panel and adjust the intensity, exposure, contrast, etc accordingly.
The app gave very good results when images of inanimate objects were processed, but when processing images of a live subject, the effect made the features somewhat difficult to make out. However, the less than impressive results may be attributed to the images captured in too little light, and not necessarily the app itself.
When you move the sliders on the settings panels, the app remembers those settings so long as it is kept running. To return them back to their median state, you will have to exit the app, and once again add the image you want to process.