Sauropod Vertebra Picture of the Week

In his post on Vicki’s new book Broken Bones, Matt told us his twelve-step process for producing stippled illustrations like this one of a crushed skull, which became the cover image of the book:

As soon as I saw that, I found myself thinking that it would look nice with some shading of the bone. Of course the existing stippling is a perfect guide to how dense the shading should be at each point, so I figured there had to be an easy way to do this automatically. There is, and this is what I whipped up in five minutes:

Here’s how I did it.

1. I loaded Matt’s image into the GIMP, my image editor of choice.
2. For some reason the crucial next step doesn’t work with greyscale images, so I converted it to RGB (Image → Mode → RGB)
3. I removed the white background, leaving it transparent (Colours → Colour to Alpha… and click OK on the default colour, white)
4. I added a new all-white background layer.
5. I duplicated the skull layer, and named it “shading”
6. I blurred the shading layer by 50 pixels (Filters → Blur → Gaussian Blur…, set the blur radius to 50 pixels and hit OK.) That gives you the shading you want, but it smudges out past the outline of the skull, hence the last two steps:
7. I went back to the skull layer, and using the Fuzzy Select Tool (magic wand) selected the contiguous transparent area outside the skull parts.
8. I went back to the shading layer and cut the selected area, leaving only that shading that’s inside the boundary of the skull.

As always with Gimp tutorials, it takes about ten times as long to explain as to actually do.

When I showed this to Matt, I rather immodestly said I was “super-happy with it”. Matt said he was “super-happy with the idea, but only regular happy with this specific execution”. He felt that the blurring was too strong, and that it should be backed off by 30-40%. So I made a new shading layer in the same way as above, but this time blurring by only 30 pixels. Here’s the resulting image:

It’s quite a subtle difference, but clear if you flip back and forth between the images (which you can most easily do by putting them in adjacent tabs of your browser). Personally, I think I prefer the 50-pixel version, since I think the shading clings rather too closely to the lines in this one, but YMMV.

Since I had both blur layers right there in the image, I thought it might be interesting to see how they look together. Here’s the result:

I’m actually rather fond of this version, but it’s a long way from the crisp, clinical feel of the original.

You can thicken up the shading by duplicating one or both of the shading layers as many times as you wish (or or course thin it out by sliding down the opacity level). Its also easy to make the shading coloured: just use Colours  → Levels, select the individual colour channels, and bring up their bottom levels to taste.

Putting all that together, here’s one I made with very dense, yellowish (bone-coloured) shading. I did it starting with the 50-pixel shading layer, upping the red output level to 200 and the green to 150, then duplicating that layer, and reducing the 30-pixel shading layer to 50% opacity.

You can play for hours with all these sliders, tweaking as you wish, thanks to the magic of layers. It’s well worth investing a bit of time to learn some of the capabilities of a program like GIMP. Matt and I are very far from wizards, but we have at least got a bit past just using it to cut out backgrounds, and it opens up possibilities.