I am interested to find a proper way of reproducing an illustration like this example.
This can be done manually by bending lines and editing stroke widths. The needed amount of work is substantial. Another answer suggests making line width modulations automatically in Photoshop. It's a good idea to save work. We try it, too. At first make an image which has
- solid white backgound
- 50% grey shape, we call it N
- solid black lines
To get acceptable sharpness the image here is 1000 pixels high.
You need later an exact copy of N. Make it into a separate layer.
Merge the white background, N and lines to a single layer, give to them Gaussian Blur
Treshold the image (Image > Adjustments > Treshold) for line width modulation:
The same can be achieved using Halftone Screen as suggested in other answer, but it needs different image color mode (=Bitmap) and quite complex settings. It creates the needed lines and the result will be sharper in low resolution, so it's well worth to be learned and tried.
The displacement needs the copy of N. Prepare the same N as the original onto a solid 50% grey background, N must have a white to black gradient fill.
Merge N and the grey layer to one and apply gaussian blur:
Save the Gradient N & Blur layer as PSD. You need the file as a displacement map.
Apply the saved displacement map (filters > Distort > Displace) to the line width modulated version of N:
If you have more blurred displacement map, you will get rounder result.
Actually I made all images in Illustrator and copied & pasted them as bitmaps to Photoshop for line width modulation and displacement map creation. My legacy Illustrator hasn't such bitmap functions. Everything can also be done in Photoshop, but to get the final vector drawing, you must trace the generated bitmap image, as said in another answer:
In Illustrator Live trace settings are critical for good results. It's very easy to get too much twists and bumps. In the screenshot you see settings which gave so good result that manual fixing is possible.
Expand the traced shape and remove exessive path anchors manually. Object > Path > Simplify distorts the shape too much, no matter what are the tool option settings.
I think this would be some plugin for illustrator (like this, not updated one scriptographer raster script) as in Photoshop same effect can be achieved by line raster.
In photoshop change image to Grayscale - use halftone screen - in shape choose line and play with frequency.
Then place image in illustrator and trace the picture.