The strokes are packed tightly together at a low opacity, making sweeping, feathery fans. In high density areas the strokes blend together to create a painterly effect, but in emptier areas the shape of the spines is made clear.
This was the closest to a gradient puzzle I've made so far - where the main strategy of solving it was to find the brightest / darkest / pinkest / bluest pieces and place them in order; I find this type of design much less engaging than having also to consider the texture, orientation, or composition of pieces to put them together.
Each jigsaw is designed and developed entirely in the browser. Above are some random outputs generated on the fly just for you – crack open the explorer for finer control of the parameters.