Anatomy of an image: Part 2 - Post-processing


Having had some questions about how I produce my visualisations, I thought it would be useful to pull apart one image, walking you through the various steps it took to produce it. This tutorial follows on from part 1 (which covered the rendering of the image in Kerkythea) and follows the post-processing in Photoshop.

Photoshop is an extremely powerful tool, the actual rendering is only half of the story - in this tutorial we're going to turn the image on the left below to the one on the right:

2-1.jpg

This is obviously a bit harder to explain in words than the rendering part, as some artistic use of the paintbrush and other tools are required, so I'll try and give a flavour of the processes used, that you can then apply to your own renders. Apologies for the length, but there's lots to cover!

Step 1: Setting up the scene & basic alterations

Firstly, we want to get rid of the mess in the background of the render, replacing it with a nice photo. After a bit of searching around the web, I found a suitable background image - a lake scene with some atmospheric lighting and moody sky.

2-2.jpg

So firstly, I used the polygonal lasso tool to cut around the building and terrace, then inserted this on top of the photo. I then scaled the render down so that it sat comfortably in the scene.

Next some basic tonal modifications to the render were needed. This is best done via adjustment layers, which preserve the original image, so if you later decide you went a bit over the top with the changes, you can easily alter or get rid of them. To add adjustment layers go to Layer -> New adjustment layer then select the type you want, or use the button on the bottom of the layers panel.

For this image I used three: a levels layer to darken the terrace, a brightness/contrast layer to add a bit more contrast to the terrace, then a hue/saturation layer to add a blue tone to the whole image. You can see the settings below. To make an adjustment only affect a part of the image (for example just the terrace), first select the area using an appropriate tool, then with it still selected, create a new adjustment layer (which will automatically be masked to the area you want.

2-3.jpg

The blue hue effect is achieved by first ticking Colorize in the hue/saturation window, then adjusting the hue slider until the right tone is found. I then set the opacity of this adjustment layer to 25% to tone down the effect to a suitable amount.

The final basic adjustment was to make the windows look a little more realistic by darkening them. I did this by first selecting all the glazed areas using the polygonal lasso, then applying a gradient fill, going from black at the bottom to transparent at the top. The opacity of the black layer was then reduced to 50%.

After basic adjustments and the addition of a background, the image looks like this:

2-4.jpg

Step 2: Adding atmosphere

Firstly, we want to add some more mist and haze to the background, which has the double effect of adding some more atmosphere, and making the focus of the image (ie the building) stand out more from the background. This was simply done with a large, soft brush. Set the colour to white and lower the opacity of the brush to something very low, then get painting. Keep it subtle, and build up layers of brushwork until you're happy. A bit of erasing around the rocks and trees gives the impression that the mist is in the background. For this image I painted over the entire of the rocky area on the right of the image to fade that out a little, and added plenty of mist in the background.

Before and after:

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I also decided to add a little suggestion of sun breaking through the clouds, mostly to add a bit of colour richness to the scene. To do this, select a light yellow-orange colour, paint in some rays using a soft brush, then set the layer blending mode to overlay and reduce the opacity until you're happy with it.

Since I had rendered the scene with wet-looking materials, I thought it would make for a more interesting image to add in some rain. Creation of fake rain in photoshop is relatively easy. First, create a new layer and fill it with light grey. Then, with your background colour set to a darker grey, go to Filter -> Noise -> Add noise. Make sure Monochromic is selected, and use a value of around 25% for the Amount slider. Hit OK, then select Filter -> Blur -> Motion blur. Change the angle to around 70 degrees, then adjust the distance until it looks good (around 30 pixels for this image). Next, give it a blue colour using the Hue/saturation window, and lower the opacity to 25%. Finally, use a layer mask (or just use the eraser) to remove the 'rain' that's on top of the building, to complete the rain illusion.

2-6.jpg

We could leave it like this, but eagle-eyed viewers will note that the 'rain' is falling onto still water... To solve this, I found a picture of raindrops on water from the web (below, left), and added a few copies of it into the scene. To blend them in, I set the blending mode of the layer to Soft Light, reduced the opacity to 75% then used a layer mask and soft brush to soften the obvious edges. The result is shown below, right.

2-7.jpg

Step 3: Plants and rocks

The next step is to add some plants to the raised planter in the bottom left of the image, and to do something about the hideous fake grass as well. For this image, to fit with the background I decided to cover the grassy area with rocks. This was a simple process of cutting out some rock sections from the original background image, and arranging them into the final image.

For the plants, I raided my conveniant library of pre-cutout plants for ones that would suit the scene and didn't have any strong lighting on them (which would have given away the fact that they weren't part of the original photo or rendered correctly). If you don't have pre-cutout plants to hand, it's worth finding some online, as cutting out foliage accurately is very time-consuming! For this scene, I arranged four plants, then painted in some shadow beneath with a soft black brush set to low opacity. Some extra shadow was needed on the side of the planter and on the nearest concrete wall too, to make it appear that the new rock is sitting against them.

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Step 4: Dodge and burn

Kerkythea has its limitations, and this step aims to address a few of them, greatly improving the realism of the final image. Firstly it lacks an effect called bloom, which is where edges appear to glow when lit by a bright light behind them (have a look at Wikipedia for more information). Since this scene is not brightly sunlit, I'm not going to do too much, but running the dodge tool at low exposure (less than 20%) over any edges facing the sun helps to suggest it.

Kerkythea also struggles with ambient occlusion. Again I don't understand this enough to explain it fully (try Wikipedia), but essentially in real life where two surfaces meet at a corner, they get slightly darker. To greatly improve the realism of our image, we can fake this using the burn tool in Photoshop. With the burn tool set to a very low exposure (15% or so), go over any joins between two faces, darkening them slightly. Keep it very subtle or the image will start to look rather odd. The burn tool is also useful for making existing shadows a bit more pronounced if it's necessary anywhere.

Before and after use of the burn tool (the effect is particularly obvious at the top of the columns):

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Step 5: Scratches and stains

Since all the textures used were tiling, we can improve the realism and interest of the scene by adding some variety to the textures, especially the concrete and timber areas. I also wanted to make the scene look a little less pristine.

Firstly I overlaid some grungy scratch/paint textures over the concrete and wood, by simply inserting them in, using a layer mask to cut them to the appropriate areas, and playing with the blending modes and opacities until they looked good. For example, for the concrete on the left of the image, the two scratch textures below were used, the first one on Multiply at 100% opacity, and the second on Soft light at 67%. The Overlay blending mode is also often useful for this sort of thing. I find it very useful to have a library to these sort of textures, which can be used to add variety to a whole range of rendered textures (wood, stone, concrete, metal etc. etc.)

2-10.jpg

Next, I added some stains to the concrete areas to fit with the general wet atmosphere and to make the building look less new. A quick search on cgtextures.com (a great website for all sorts of textures and other useful bits) found the image below left - for each area of concrete I then inserted this image, distorted it to fit the perspective of the area in question, then set the blending mode to Soft Light and lowered the opacity to around 70%. In theory of course I could have rendered these details in, but it's far far easier to just add them in Photoshop afterwards! The image on the right shows the completed concrete texture.

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Step 6: Finishing touches

We're almost finished, but there are still a few more things to add and change. Firstly, the glazing in the scene would in reality reflect the surrounding landscape, but because our surrounding landscape here was added in Photoshop afterwards, the windows look wrong. This is of course easily fixed - dublicate the background, flip it horizontally, layer mask it over the areas of glazing (I got lazy at this point and ignored the thin blinds over the windows), then set the blending mode to Overlay and reduce the opacity.

Next, I decided to add some fallen leaves onto the terrace for a bit of foreground interest. Again, this was nothing complicated, I simply found a suitable leaf picture on the web, distorted it to fit the perspective, then dublicated it lots of times, altering each one's size and rotation a little so it wasn't obvious that they were the same leaf.

So here's how our (almost) finished scene currently looks:

2-12.jpg

The final stage is the most dramatic, and isn't for every render, but in this case I think it lifted it into something but better. I learnt the following technique from a tutorial by Alex Hogrefe - I would highly recommend checking out his website at www.alexhogrefe.com, as he produces some amazing images using a similar workflow to mine (i.e. Sketchup -> Kerkythea -> Photoshop) but with better results, with lots of well-written tutorials to explain how he does it. He also has some great tutorials for other presentation styles, including several that don't require any rendering at all (just some careful Photoshopping).

Only do this stage once you're happy with everything else. First create a new layer, then hit Ctrl + Alt + Shift + E to copy all visible layers to the new one (a useful shortcut to learn!). Bring up the Hue/Saturation box, check Colorize, and change the hue to a blue tone (and reduce its saturation a little). Golden tones or just monochrome can also work well, depending on the image you're producing - for my atmospheric, moody one, a blue tone seemed appropriate. Next go to Filter -> Blur -> Gaussian blur. I set the radius to around 6 pixels for this image, but it's worth playing with a few settings. Finally, set the blending mode to Overlay, and reduce the opacity to around 60% and that completes the effect. It's hard to explain what happens, but it definitely look great! Although the realism is reduced, I find this process makes my renders look much less precise, and also helps to blend render and background together.

Conclusion

Here's the final image, I'm pretty pleased with it!

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So that's it for this series of tutorials, I hope they prove useful or inspiring! I learnt much of the techniques covered simply by fiddling with Photoshop for a long time (and a healthy dose of learning from others of course) so the best advice I can give is to simply play with Kerkythea and Photoshop as much as possible and keep practicing!

Submitted by Nick Howlett82 Articles
Published on Tuesday 13 September 2011View Profile