The high level GUI toolkits that I looked at (GTK+ and Qt) were very bloated with features that would not come in handy for my use case, and frankly, they were difficult to use. Doing simple tasks in Xlib was simpler. But I did quickly notice a lot of limitations with Xlib such as no transparent pixels which means no Anti aliasing and the lack of ability to load in True type and Open type fonts. I did come across the Xrender library (also by XOrg) but couldn't find any documentation for it. Also doesn't really help that pretty much any documentation for Xlib is dozens of years old. Which (C) GUI libraries do you have in mind?