A raytracer built during my the Fall 2018 semester of UIUC's CS419: Production Computer Graphics. Built in C/C++, and supports OpenCL (realtime raytracing) on compatible devices with limited rendering features.
I followed these instructions here (seems that MacOS OpenCL headers for C are included, but those for C++ are not) and used [this code sample](from https://gist.github.com/ddemidov/2925717) to get OpenCL working. I had to copy a distribution of the OpenCL C++ headers, cl.hpp, to my /System/Library/Frameworks/OpenCL.framework/Headers folder in order to include OpenCL bindings in my project.
I ran into some issues during this, but I was able to get this working after installing a new version of g++ through Homebrew (g++).
I am using SDL for drawing graphics. You can use the steps here to setup SDL on MacOS.
I use libpng to convert the SDL texture I render to into a PNG. See SDLRenderer::save_png(string filename). The render texture is drawn to an SDL_Surface, where the pixel data can be read and saved.