constant pixel width for any scale I have a standard 3d camera/projection/model setup that renders rectangles with a WebGL shader. 

Users can zoom out and fly anywhere. 

I want to make sure that regardless of the scale, width of the rendered rectangles is a constant value (e.g. 2 pixels). 

What is the most efficient way to do so?

This is very similar to SVG's non-scaling-stroke vector effect: https://www.w3.org/TR/SVGTiny12/painting.html#NonScalingStroke
Found a solution. 

In the vertex shader I project position from model to the clip-space, then from clip-space to screen space, adjust the width in screen pixels, and the project back to clip-space, which ultimately becomes gl_Position value.