Anatomy of an image: Part 1 - Rendering


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 will cover the rendering of the image (including some more advanced materials and techniques), which was done in Kerkythea after modelling in Sketchup. Part 2 will cover the post-processing in Photoshop.

So here's the image I'll be dissecting:

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Set-up in Sketchup

A good render needs a good model first - this one was downloaded as part of a visualisation competition, from www.ronenbekerman.com. I started by picking the scene I wanted to render, then tidying up the model, deleting anything that wouldn't be in the final image (to speed up the rendering time). I had already decided that I wanted to set the house next to a lake, so built a terrace with slight raised edge. A few choice furniture items from Sketchup warehouse completes the scene. Here's how it looked in Sketchup:

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Next we need to set-up ready for rendering in Kerkythea. For those who haven't heard of Kerkythea, it's a relatively simple, freely available bit of software, that nevertheless can be used for some fairly complex rendering - it might help for this tutorial if you already know the basics; conveniently I've already written a basics tutorial, which you can see here.

Kerkythea works by splitting the model by Sketchup material - i.e. anything that is painted with a certain material will be rendered as the same material in Kerky. For this image, I wanted some fairly rough textures, so I painted most of the house with concrete (don't worry about the Sketchup texture you use looking good, as it will all be replaced in Kerky later). The soffit of the overhanging roof and the balcony are painted with brown (to eventually become timber) - I used several different browns, to allow different parts to be rendered with different timber textures. This is especially important where the timber wraps around the eaves of the overhang, as a different texture is required in the rendering to make it look like the wood realistically wraps around the eaves. As it happens, I decided eventually to replace one of the timber areas with white render. You can see the different textures in the image below (green became the white render):

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Hit the export to Kerkythea button, and we're ready for the fun bit.

Material set-up: concrete

First we need to set-up all the materials in the scene - I started with the concrete, which I wanted to appear rough, with formwork marks still showing. None of the material libraries I have installed had exactly what I wanted, so I built the material from scratch, using a high-res texture found on the web.

Once the exported model is in Kerky, hit the V key to make things a bit easier to see, then click on the concrete in the model (it will be highlighted in yellow). Right click the relevant material from the left-hand pane (which will have a yellow star by it) then go to edit material. Right click where it says Matte/phong then click Reset.

First in the Reflectance area, right click where it says Diffuse, then click the second box along (Add bitmap). Browse to the main texture image, then hit ok - this is the main channel that controls the look of the texture. See my basics tutorial for more detail about basic material creation.

Next we want to make the texture appear a bit more realistic, adding in some shininess and bumpy-ness. Both of these techniques require some trial and error to get the best values for the various parameters, so a few test renders are usually needed.

Shininess can be done in several ways, but here we're going to use the specular channel. First, right click where it says Specular (in the Reflectance area), then click the first box along (Add colour) and pick white from the colour selection wheel. To make the specular render properly, we then need to tick the Specular Sampling box at the bottom of the Reflectance pane. Next, we want to make the shininess less precise, so change the Shininess value to a much lower number - 20 in this example. Finally, we don't want the concrete to appear too bright so, making sure Specular is selected in the Reflectance pane (a Weighting box appears lower down the screen), lower the Weight to around 0.2.

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Bump mapping is a crucial rendering technique, since very few materials are truly flat, but all will render that way unless some bump is added. To do this we need to create a bump map for the material we are using - essentially a monocrome image tells the software where to make the material seem higher and lower; black is lower, white is higher. These can be made very quickly in Photoshop, by first desaturating the texture, then adjusting the Levels to boost the black and white tones. Below are the two textures used for this material: on the left is the diffuse image, on the right the bump map image.

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To add a bump map to a material, right click on Texture in the Bump Mapping #0 area, and add a bitmap as you would a normal texture image. The Strength parameter below sets how strong the bump is - some trial and error led to a value of 0.37 for this texture.

Material set-up: terrace slabs and timber

For the terrace I chose to use a mixture of stone slabs and timber. At this point I decided it would make an imteresting final image to introduce some weather - mist and rain in this case. This led to developing a way of making the materials look wet, via a long process of fiddling around with various settings.

For the slabs, I again built the texture from scratch. Following the same process as for the concrete, I added a tiling diffuse map, then created a bump map and added that in (using a Strength of 0.68 this time). The wetness was achieved by a mixture of Specular and Reflection.

First, right click on the Specular channel (as we did above), but this time select the third box along (Add Procedural). Click on Perlin Noise Texture then OK. This simulates a wavy reflection, as if there's water lying on the surface of the material. Next, add a white colour to the Reflection channel, then making sure it's still selected, adjust the Weighting to 0.45 (we don't want it ridiculously reflective). Finally, tick the Specular Sampling box, and leave the other settings as their default values.

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The timber decking was based around a timber plank texture I downloaded and installed previously, with the same specular and reflection settings as the slabs applied, to again make it appear wet.

Material set-up: everything else

Pre-downloaded textures also provided the rest of the timber textures used in the render. The only modifications needed were scaling and rotation of the textures to fit the model properly. This is done by selecting the texture on the model, then going to Tools > Bitmap Coordinates from the top menu. On the Bitmap Coordinates pop-up that appears, make sure the second box along is selected (Cubic Projection), then you can alter the scale and rotation of the texture using the sliders below.

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Next the glass. This is based on a thin glass material from a pre-downloaded library, which I modified to make it a bit more reflective. This is simply done by altering the Index of Refraction of the thin glass, up to 2 in this case.

For the small bit of white render, I first applied a rough plaster texture I already had downloaded. After reducing the bump map strength, I then added a white colour over the top of the existing texture in the Difffuse slot, to make it a bit whiter whilst still keeping the texture. With Diffuse selected, lower the strength of the white component (to 0.16 in this case) to avoid it being far too white and glaring in the final image.

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The final materials were all simple - a preset shiny metal for the railings and chairs, dark brown for the window frames etc. I ignored the grass and bits inside the planters as that was all to be added in Photoshop later.

Render settings

First we need to play with how the scene is lit, so go to Settings > Scene on the top menu. There is only one light in the scene and that's the sun. First I want to make it stronger, so up the Multiplier value, to 3.8 in this instance. To make it more realistic we also need to change the colour of the light - click on the white box in the Radiance area, then change this to a very light yellow (subtlety is the key here). Finally, a great way to make renders more realistic is soft shadows, although they do significantly increase render time. To do this, make sure the Soft Shadow box is ticked, then adjust the Radius slider as appropriate. Again this needs some trial and error - I used a value of 100, more close up scenes such as interiors will require much lower values.

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If the shadows were correctly set up in Sketchup, then they will render right as well, but if they are wrong, go to Settings > Sun and Sky and enter the correct time of day and year.

As I had already decided that I wanted a moody final render, I didn't want the render to appear like it was brightly sunlit. An easy way to change the lighting of the scene is to insert a Global, which essentially places a sky photo all the way round the model, and the colour of the lighting is dictated by that sky photo. I have a whole selection already downloaded - for this render I used an overcast preset, which is added by going Insert > Global then finding the image you want.

Finally, we need to check the Render Settings (go to Render > Setup on the top menu). For the final render, I made sure to increase the quality of the soft shadows to Ultra-fine, otherwise you end up with poor quality shadows with lots of noise - this massively increases render time though so be prepared for a wait (unless you're blessed with a much better computer than me).

So that's it. Go to Render > Start to set the render going, making sure you put in a sensible resolution (my final one was done at 4000x2000 - it's always best to render bigger than you need then shrink down for printing etc).

Conclusion

So that concludes the first part of this tutorial series. The completed render is shown below - doesn't look all that special yet, but most of the magic happens in Photoshop, which is covered in part 2.

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Submitted by Nick Howlett82 Articles
Published on Sunday 11 September 2011View Profile