This paper introduces a novel content-adaptive image downscaling
method. The key idea is to optimize the shape and locations of
the downsampling kernels to better align with local image features.
Our content-adaptive kernels are formed as a bilateral combination
of two Gaussian kernels defined over space and color, respectively.
This yields a continuum ranging from smoothing to edge/detail preserving
kernels driven by image content. We optimize these kernels
to represent the input image well, by finding an output image from
which the input can be well reconstructed. This is technically realized
as an iterative maximum-likelihood optimization using a constrained
variation of the Expectation-Maximization algorithm. In
comparison to previous downscaling algorithms, our results remain
crisper without suffering from ringing artifacts. Besides natural images,
our algorithm is also effective for creating pixel art images
from vector graphics inputs, due to its ability to keep linear features
sharp and connected.