Winter wonderland!
That was a pleasant short film, with visuals that really helped accentuate the movie's wholesome and whimsical theme. First, the excellent use of colors, light, and shadow helped to set the mood of a dreamy, whimsical winter's night. Also, the animation was good (not great), but without being too showy. Finally, the detailed backgrounds added to the movie's quality.
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Content-wise, this movie has a wonderfully simple yet pleasing feel to it. What a pleasant revelation to discover Nim's relative size! Watching the movie a second time, it's clear that the author did an excellent job of hiding Nim's size until he reaches the cabin - which has logs thicker then he is tall. The fact that he does odd-jobs for the family pro-bono is a really heartwarming bit.
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It's worth mentioning too that the music is excellent. It even has appropriate transitions between scenes. The only thing that detracts from this movie is the inexplicable "pink camel desert" bit near the end. It completely shatters the cool winter night effect. Other than that, perhaps Nim's outfit could have a more textured and detailed look, rather than the subtle-gradient look. This would better fit in with the richly detailed backgrounds.
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All in all, an excellent job. This movie sets out to tell a simple winter's tale, and succeeds wonderfully due to excellent presentation.