I spent way too long trying to figure out this problem with dlib while using the Python face_recognition library that wraps it, and since I couldn't find anyone giving the correct diagnosis and solution online, I'm posting it as a public service to the next person who hits it. Here's the error I was getting: RuntimeError: Error while calling cudaMallocHost(&data, new_size*sizeof(float)) in file /home/jonathan/Projects/dlib/dlib/cuda/gpu_data.cpp:211. code: 2, reason: out of memory Eventually I gave up and switched from the GPU model ("cnn") to the CPU one ("hog"). Then I started getting errors about RuntimeError: Unsupported image type, must be 8bit gray or RGB image. The errors persisted after adding PIL code to convert to RGB. This one was easier to track down on Google: it happens when you have numpy 2.x installed, which is not compatible with dlib. Seems like something along the way should give a warning about that! At any rate, with numpy dow...