face detector fails on large images?
If the faces in your images are very large, then the face detector may not be able to detect the face using the default smallestFaceHeight parameter.
The face detector has a detection scale range of about 5 octaves. Ex. 40 pixels yields the detection scale range of ~40 pixels to 1280 (=40x2^5) pixels.
If you are dealing with very large images, or with dynamic input images, it is best to set the
smallestFaceHeight parameter to -1. This will dynamically adjusts the face detection scale range from image-height/32 to image-height to ensure that large faces are detected in high resolution images.