The light setup was quite simple and you can see it in Fig.05. Firstly there is the key light (1), above and behind the hand. It actually acts as a rim light as well. Then there is a fill light (2), positioned to match the lighting from the photo. There is a little trick you can use here. Create a sphere roughly in the center of the image and use the Place Highlight tool to put the light in the right place, corresponding with the specular highlights on the skin. This way, matching things like the warm light on the tip of a middle finger is an easy job (3). Finally there's an additional rim light for the left-top facing parts of the fingers (4 - 5). All the lights are mapped with gradients, either linear or radial. This way the reflections they produce look more studio-like.
I used GI and a diffusely-convoluted HDRI map to provide additional fill lighting, and a high-resolution HDRI to generate nice reflections. The reflection map has been rotated, so the reflections show up in the right places. Color mapping was set to Exponential, so it doesn't blow out the highlights.
To see the progress, let's take a look at the sequence of WIP images I saved during the project (Fig.06). See if you can spot the changes between the images.
When I was happy with the preview renderings I started working on a high resolution render. I had a bit of a setback here because in full-size a slight lack of surface detail became apparent. This was fixed in two ways. First I added another layer of dirt to most of the materials, with a fine texture of spots and scratches. I also added to or increased the strength of the bump mapping in places. The second way was another render pass, containing three various textures in the R, G and B channels, occlusion, spots and anisotropic specular, modulated by tiled scratches, all to be added in compositing. All in all these were subtle changes, but important when you look at the image when it is large and at full size.
There were many little steps involved with compositing this image. The first one is masking. I used a layer mask to hide parts of the CG fingers that should be occluded by real elements. For a still image it's the fastest way, but this is not the case for an animation. I also used the Clone Stamp tool to remove the portions of the real fingers visible behind the CG ones.
The next crucial step was matching the brightness and contrast levels in both the photo and CG elements, and matching the colors. This was done using Curves adjustment layers. I added some contrast to the photo and boosted the shadows on the CG a bit.
Another important thing is matching the depth of field. Although subtle, blurring was present and needed to be taken into account. I rendered a ZDepth pass and fed it to the Lens Blur filter. It took a few tries to get the blur amount right, but in the end it matched. The same treatment was necessary for the VrayReflections pass, which I added on top to make the reflections "pop" more.
Then came the polishing tweaks, like adding some blurred smoke in the background, adding a bit of glare in the top-right corner and removing skin-toned finger reflections from the scalpel. I also added an orange logo to the handle (to tie the photo and CG in better, since now both had strong orange accents). I added the additional rendered dirt and specular passes to the CG fingers and painted in some shadows (mostly, the shadow of the scalpel on the CG fingers and darkening the bottom finger behind the scalpel). Finally I painted in greenish fringes/glows on the edges of the CG parts.
To tie both parts better, I took a look at the Channels palette and noticed there was quite a bit of noise in the blue channel of the photo. Needless to say, I added something similar to the blue channel of the CG part. You don't even notice it unless you look into Channels, but it's there. I also added some grain over all the channels later on.
The boundary between flesh and prosthetic is always a challenge. I didn't want anything gross or ugly, just a clean, painless joint (it was very important for me that the cyborg addition looked comfortable). Fortunately the boundary was quite small in relation to the rest of the image. Still, the best way to make it work was finding an existing feature that I could use to "anchor" the prosthetic; in this case a skin fold in the middle of the first finger segment. Then it was just a case of darkening the skin in the whole area and painting in some shadows and highlights, both on the skin and on the CG part. In the end the boundary didn't draw too much attention to itself, which is what I wanted. This method is (again) almost completely useless for animation, but very fast and efficient for still images.
A final touch was the global color correction. This is a subtle modification of Curves, with the blue curve being shaped into a very shallow S shape, which adds a hint of color to the highlights and shadows separately. I also added some vignetting to darken the corners of the image (Fig.07).