Open Source C Image Viewers for Mac

gPhoto is a program and library framework that lets users download pictures from their digital cameras. The libgphoto2 library gives you access to hundreds of models of digital cameras on several platforms. GIT repo and Issue tracker lives at https://github.com/gphoto/ these days.

Luminance HDR is a complete suite for HDR imaging workflow. It provides a wide range of functionalities, during both the fusion stage and the tonemapping stage. Its graphical user interface, based on Qt5, runs on a variety of platforms, such as Microsoft Windows, Mac OS X 10.9 and later and several Unix flavors (Linux, FreeBSD and others). Input images can be supplied in multiple formats, from JPEG to RAW files. In the same way, output can be saved in many different formats as well, from JPEG to TIFF (both 8 bit and 16 bit per channel), enabling all the power of your post processing tools.

ImageStone is a powerful C++ class library for image manipulation. Its features include load, save, display, transformation, and nearly 100 special image effects. It can be used cross platform (includes Windows, Linux, Mac), and especially under windows it can be used as a DIB wrapper class.