  We propose a new automated digital painting framework, based on a painting
agent trained through reinforcement learning. To synthesize an image, the agent
selects a sequence of continuous-valued actions representing primitive painting
strokes, which are accumulated on a digital canvas. Action selection is guided
by a given reference image, which the agent attempts to replicate subject to
the limitations of the action space and the agent's learned policy. The
painting agent policy is determined using a variant of proximal policy
optimization reinforcement learning. During training, our agent is presented
with patches sampled from an ensemble of reference images. To accelerate
training convergence, we adopt a curriculum learning strategy, whereby
reference patches are sampled according to how challenging they are using the
current policy. We experiment with differing loss functions, including
pixel-wise and perceptual loss, which have consequent differing effects on the
learned policy. We demonstrate that our painting agent can learn an effective
policy with a high dimensional continuous action space comprising pen pressure,
width, tilt, and color, for a variety of painting styles. Through a
coarse-to-fine refinement process our agent can paint arbitrarily complex
images in the desired style.
