I am using Int#frame_index with a starting values other than 0, but it is still counting from 0. I tested this in def tick proper to verify.
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I just updated my OP so you can see what I'm looking at. Take a look at https://github.com/kniknoo/oo_dragon_demo/blob/master/lib/sprite.rb , line 34 will point you to the workaround above and the default behavior below.