  Large language models (LLMs) have the potential to impact a wide range of
creative domains, but the application of LLMs to animation is underexplored and
presents novel challenges such as how users might effectively describe motion
in natural language. In this paper, we present Keyframer, a design tool for
animating static images (SVGs) with natural language. Informed by interviews
with professional animation designers and engineers, Keyframer supports
exploration and refinement of animations through the combination of prompting
and direct editing of generated output. The system also enables users to
request design variants, supporting comparison and ideation. Through a user
study with 13 participants, we contribute a characterization of user prompting
strategies, including a taxonomy of semantic prompt types for describing motion
and a 'decomposed' prompting style where users continually adapt their goals in
response to generated output.We share how direct editing along with prompting
enables iteration beyond one-shot prompting interfaces common in generative
tools today. Through this work, we propose how LLMs might empower a range of
audiences to engage with animation creation.
