1. In a digital medium environment to generate improved training data to train a model to label pixels, a method implemented by a computing device, the method comprising: receiving, by the computing device, an image including a structured object representation configured according to a markup language or portable document format, the structured object representation defining a hierarchical relationship of a plurality of objects, one to another, to corresponding pixels within the image; selecting, by the computing device automatically and without user intervention, an object from the plurality of objects of the structured object representation; displaying, by the computing device based on the selecting, the corresponding pixels of the selected object in a user interface; labeling, by the computing device, the corresponding pixels based on an input originated through user interaction with the user interface, the input labeling the displayed object; and training, by the computing device, a model using machine learning to label pixels, the training based at least in part on the labeling of the corresponding pixels for a plurality of said images.