Incidentally (since I happen to be working on just this task right now) you can do some very nice things with procedurals used to control transparency because they work well at any distance.  To get something this fine with a bitmap would require a really huge bitmap, or I suppose you could tile a smaller one as AndyStaples is doing.  This is a Tile node drawing white lines on black background.  While it might be more accurate to use the Weave node I had problems getting the space between fibres to be opaque where it neaded to be opaque, that node works much better for bump maps.

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