Once you are satisfied with the line work, you can dispose of the very first layer, the original image. After deletion, flatten the image, converging ...
This Instructable shows how to take a portrait and turn it into geometric art. This process can also be applied to animals, figures, and landscape. The image I am using is a stock photo and I claim no ownership or profit from this photo, I am merely using it a tutorial piece for learning purposes. This complete process was made at Techshop.
Before starting in Adobe illustrator, I opened the file in Photoshop. I then chose the 'adjustments' feature, and chose 'levels.' Under 'output levels' use the bar to lighten up the image, making it easier to see the lines versus the shapes and colors of the original image.
The image has now been opened in Adobe Illustrator. I started with the eye, as a way to start with the main feature of the illustration, and work my way out.
Slowly broadening out now. I am not solely focused on anatomical correctness. While not straying too far from anatomical features, I also chose to connect certain segments, merely because I thought they were interesting. However this is my preference, and people can enjoy experimenting with different techniques.
Continuing to work my way out and around the facial features and face itself. After I am more satisfied with the line work I go back to certain areas, deleting lines, and or moving them to connect with other segments. Just until I am satisfied with how the picture looks.
Once you are satisfied with the line work, you can dispose of the very first layer, the original image. After deletion, flatten the image, converging all the layers. To go further with your illustration, you can also use the Paint Bucket Tool to cover specific geometric shapes, and contrast colors.