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What Industrial Design Renderers Know that the General Public Doesn't: The Craik O'Brien Cornsweet Illusion

By Rain Noe - December 11, 2013

in Object Culture

Image by Dale Purves (see info below)

One of the things I loved about seeing Scott Robertson's presentation on rendering tricks at Autodesk's CAVE Conference: The man still renders in Photoshop. I cut my ID teeth rendering bottles in Adobe's flagship product, and it's nice to see that not everyone has completely gone the 3D route.

When you're manually (albeit digitally) laying down gradations and layers, you quickly learn how much black you have to put into something to make it look white, and how much white you have to put into something to make it look black. The optical illusion up above, which has recently gone viral and is shocking to anyone who's never done an ID rendering, is an excellent example. The top chiclet is black and the bottom is white, right? Well, not if we look at it after masking off most of the drawing:

Yep, the top and bottom chiclets are in fact the same exact shade of grey. It's the highlights, shading, drop shadow and that junk in the background that fools your eye into assigning different values to it. While they never taught us this in ID school, the actual name of this phenomenon is the "Craik-O'Brien-Cornsweet illusion." Taken by the illusion, Slate even dug up a video demonstration of it:

That image above that's gone viral, by the way, is the work of Dale Purves. Purves is no ID'er; the man is Director of the Neuroscience and Behavioural Disorders program at Duke-NUS Graduate Medical School. His Purves Laboratory studies "visual perception and its neurobiological underpinnings."

Am hoping some of you Photoshop renderers will comment with links to renderings demonstrating your Craik O'Brien Cornsweet skillz.

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jans

10 years ago

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Of course what the woman does with the squares is an imaging trick. You cannot move something from shadow to light without changing its hue. But that is not the point of the video. You could have demonstrated it with the Photoshop colour picker but a lot of people not familiar with it wouldn't have understood. The point is that the squares have the same hue before she moves it. The way they chose to present it, is a bit unfortunate though.
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Mathieu

10 years ago

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I agree with Paul.
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Paul

10 years ago

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I've got to say, as an industrial designer this 'illusion' really annoys me. You can't apply a 2D illusion theory on a 3D image (no matter how poor) or at least not without properly setting the rules - the version I saw asked if the top object was the same colour as the bottom object - Um, no it's not.

Of course the center of each black and white object has the same value! The black object is in full illumination, and the white object is in shadow. It's not a fair test.

Don't get me wrong, I can totally understand and appreciate flat illusions (where a light square inside a dark field looks lighter than a dark square inside a light field) but once you add shading and an apparent light source, it's no longer an illusion. This image is a virtual representation of what in the real world would be black (or grey to my eyes) and white. Just because you can take a colour picker to it in photoshop doesn't make it the same colour in 'real life' (apparent).

Which brings me to the video. It's fake, or at least the shadow area must have been augmented by extra colour (or a little AfterEffects trickery). It may 'work' as the still image that the video was based on, but as a video it's set up, because anyone who can render, will tell you if you set it up like it looks (as a simple checkerboard with a cast shadow), the light squares will always be light and the dark squares will always be dark. It's only the shadow that makes the light squares (under shadow) the same shade as the dark squares (under the light) - once again when you take a colour picker to it as a still image in photoshop.
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