Sliced
From SarahWiki
Sliced
The idea behing this photo has been rattling around inside my head for many months now, but I'm happy to say it's now finally 'out there'. It was held up by not having any suitable lights that I could use with my Better Light (being a scan back, it needs continuous lights, not strobes). So, as I finally got myself some lights (a Westcott Spiderlite kit -- full review to follow once I've played with it a bit more), I took the plunge and used them in anger.
This is the first article in a new series of 'making-of' essays that talk about how a particularly complicated photo was constructed. I tend to do things my own way -- I'm sure that there will be plenty here to disagree with and do differently, but that's partly the point -- I wish more people would talk about how they construct their photos, so I'm leading by the nose here.
Contents |
Concept
This photo follows another oddball image of mine, Chicken Egg Problem.
In both cases, I wanted to go for an image that felt almost 'too' real, twisted in such a way that it appears ordinary at first glance, then when your brain starts to process what you're actually seeing you realise that the image is semantically impossible. With CEP, people start wondering if I have a very, very sharp knife, or my own personal space laser or something. In this case the runny bits of the egg along with the cracked edge of the shell is all real, but the rest of the image is entirely CGI, created in LightWave and then composited in Photoshop. Sliced is similar, though this time more of the image is real -- the chopping board, knife and backdrop are all real, but the glass and liquid is all CGI.
How I did it
The first step was to set up the view camera and just generally fiddle around to get a composition that worked. I started with a 90mm, but found the perspective a bit too severe, so I swapped to the 150mm. There was very little depth of field -- this is a typical issue with large format, and it certainly bit me today. I used quite a lot of front tilt and managed to get the surface of the board pretty sharp across its whole area. I allowed the top left edge to blur slightly -- initially this was an accident, but I decided to keep it because I liked the look and I thought it would emphasise scale well.
The first thing I did was get the chopping board placed on a large sheet of cream-white mat board just where I wanted it:
The next thing I wanted to do was place the kitchen knife and get the lighting just right. After some fiddling, I realised I was stuck -- I could either have the knife handle and the front edge of the board look right, or everything else look right, but not both. So, following time-honoured tradition, I cheated.
First I shot the composition lit for the board:
Then I adjusted the lights, and made another exposure:
This one made the knife handle look good, but blew out the blade somewhat. I also liked the shadow along the front of the board.
The last actual photograph I needed was an alignment target so I could (relatively) easily line up the CGI scene with the real scene (the box is lined up with the back and right edges of the board):
This last image shows up the effect of the huge tilt -- the box is sharp at the bottom, but gets less sharp the higher above the board surface it gets. In fact, if you look closely on a big version of the images with the knife in them, you can see that the top surface of the knife is actually slightly unsharp too, again for the same reason. I didn't make a second shot with altered focus because comping this kind of thing can be a total nightmare -- when you adjust focus, you also scale the image, which can often be less than 'fun' to correct later.
CGI
So much for the photography. The hard part of this project was certainly the CGI. Compositing computer generated and real images is actually really quite difficult -- Hollywood makes it look easy, but if you ever try it you'll realise what an amazing job those people do. I started by taking a ruler to all of the real objects, taking precise measurements and making detailed notes. My approach was to model the cutting board reasonably accurately (to the nearest millimetre) so that reflection and refraction would look plausible. The first thing I needed was a texture map for the board's surface. Rather than shoot the board flat-on, I decided to take the texture from one of my existing images so I could minimise changes due to differences in lighting. This is actually pretty easy in CS2 -- I used the crop tool in perspective mode, dragged the corners out to the edges of the wood surface, with the crop tool set for 200dpi and the exact dimensions in mm of the wood. Clicking crop again gave me this:
Neat trick, that. Crop is a surprisingly powerful tool.
The first CGI model I built was for the board. This was pretty straightforward, with no rocket science required. Next came a glass -- I started with a cylinder of the correct outer dimensions, then chopped out a capsule-shaped dip with constructive solid geometry (CSG). I then trimmed the capsule to make the liquid, and then composited the two models together into a single object. After loading it into Lightwave, a considerable amount of tweaking resulted in something plausible-looking. Simulating glass and liquids is pretty tricky to get right, so it took a lot of experimentation. I used pretty much all of the fancy rendering options -- ray traced shadows and reflections, radiosity, caustics, refraction -- you name it, it was probably in there. The 'chopped up' version was made by taking a copy of the original object, making a series of thin plates, then using CSG to slice the object against the planes. Once that was done and cleaned up slightly, it was a relatively simple matter to select and arrange the slices.
Rendering took a while. I ended up rendering the intact glass separately from the sliced-up glass -- this was partly to save CPU time, and partly because I couldn't run the rendering in such high quality for the cut up pieces because it would just have taken too long.
Intact glass with all the rendering options turned up.
The chopped up glass is still shown because it needs to be there for the reflections and refraction in the intact glass to look right.
Same trick again, this time with the chopped-up glass turned up.
Note that the pieces lying flat look a bit dark -- I fixed that in the comp later, because it was much easier than altering the lighting.
Compositing
The first thing I did was composite the two CGI frames together, so I only needed to deal with a single image in the main composite later:
Note that I've fixed the darkness problem here.
The next step was to composite the two shots of the knife on the real board:
I also took care of the top right hand corner at this point.
This now brought me to the point of doing the final composite. The alignment between the two images wasn't bad, but wasn't perfect either, so I used the mould tool to tweak the CGI image to perfectly match the real image. Once that was done, it was now down to laboriously painting a mask by hand, and then adding some slight shadows. Finally, a levels layer with some minor tweaks sorted out contrast, resulting in the final image (see the top of the page).
Final composite image
Conclusions
This was a tough project, but it was completed in less than a day, so it's certainly not the hardest thing I've attempted. I'm not sure what will come next in this series, but I can be pretty certain that it's going to annoy me until I get around to making the picture!
