Hi everyone!
I want to produce bas/low-relief of towns, monuments etc from an image. I’d like to 3d print them after that. I’ve been told you can do it with a purpose made black and white image, which you can then transform in 3d in Zbrush. I’d like to find a tutorial for this process. I’m a “traditional” artist, I use Photoshop everyday in my workflow and I’ve just started to learn Zbrush. Thanks to everyone who takes the time to read this! And sorry about the English 
I think this one will be useful. https://www.youtube.com/watch?v=eGlzP1y0AO0&list=PLMjnnUF3eJFf5H8r3YyXYvGXUEWl52vQa
Most of the videos/ articles I’ve found on bas-relief via ZB just pull relief sculpture straight from a supplied photo.
This works on a basic level, but as the alpha treats light colours as high relief and dark colours as low relief, any form of cast shadow on the reference image comes through as a deep pit in the model.
This might not matter at small size (I have got away with it in the past), but as you get larger and deeper, you’ll find that the final relief image only makes visual sense from a front on viewpoint. As soon as you look at the model from an angle, the pits/peaks left by the cast shadows from the reference image start to interfere with the sculptural form.
As bas-relief relies entirely on light to create it’s sense of depth, any unwanted relief ruins the effect.
All my work is bas-relief, and I use alphas a lot, but only to apply surface texture - not to create the base profile of the model.
When I use a reference photo as an alpha, I usually spend a good few hours editing out the cast shadows, and balancing the contrast, so that the image doesn’t leave me with deep holes or nasty peaks.
I’ll build the overall profile of my sculpture from scratch, then adjust the alpha texture to fit. Sometimes, it’s actually easier to build the base profile, take an alpha render of it from ZB, line it up with the detail alpha in Photoshop, then bring the whole lot back into ZB, and generate a model from there.
Recently, I’ve been playing around with Substance B2M (bitmap to material), to make relief textures from reference images. It can remove cast shadows on the fly, which has saved me loads of time!
I make no apologies for being a purist, having spent 20 years as a bas-relief sculptor (digital and material), so I’ll always bemoan the ‘auto relief’ methods! The best way to understand ‘real’ bas-relief is to read Edouard Lanteri’s books on sculpture. You can often find the 3 volumes as a single publication, but vol 2 is the one that deals specifically with bas-relief.