View source for Cornell box

{{short description|Computer graphics 3D reference model}}
{{about|the graphics rendering test|the sculptures|Joseph Cornell}}

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| [[File:Cornell_box_render_by_Cornell_University_Program_of_Computer_Graphics.jpg|300px]]
| Rendered by [[Cornell University]]<ref group="𝓲">More info in the [[#Synthetic image]] section and the file description.</ref>
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The '''Cornell box''' is a test scene designed to evaluate the accuracy of [[rendering (computer graphics)|rendering]] [[software]] by comparing a [[3D rendering|rendered image]] with a [[photograph]] of a real-world model under the same [[lighting]] conditions. It has become a [[List of common 3D test models|commonly used 3D test model]] in [[computer graphics]] research.<ref>{{Cite journal|last=Niedenthal|first=Simon|date=2002-06-01|title=Learning from the Cornell Box|journal=Leonardo|volume=35|issue=3|pages=249–254|doi=10.1162/002409402760105235|s2cid=57565464 |issn=0024-094X|url=http://urn.kb.se/resolve?urn=urn:nbn:se:mau:diva-1766 }}</ref>

The box was created by Cindy M. Goral, Kenneth E. Torrance, [[Donald P. Greenberg]], and Bennett Battaile at the ''[[Cornell University]] Program of Computer Graphics'' as part of their research on [[radiosity (computer graphics)|radiosity]] and [[Diffuse reflection#Interreflection|diffuse interreflection]]. Their findings were published in the paper ''Modeling the Interaction of Light Between Diffuse Surfaces'', presented at [[SIGGRAPH]] '84.<ref name="CornellBoxHistory">[http://www.graphics.cornell.edu/online/box/history.html History of the Cornell Box]</ref><ref name="CornellBox1984">Cindy M. Goral, Kenneth E. Torrance, Donald P. Greenberg, and Bennett Battaile. ''[https://dl.acm.org/doi/10.1145/964965.808601 Modeling the Interaction of Light Between Diffuse Surfaces] {{Webarchive|url=https://web.archive.org/web/20100627091436/http://design.osu.edu/carlson/history/PDFs/goral.pdf |date=2010-06-27 }}''. SIGGRAPH 1984.</ref>

== Reference Model ==
[[File:Cornell Box with 3 balls of different materials.jpg|thumb|300px|Cornell box with three spheres illustrating different interactions of light and surfaces.<ref group="𝓲">Picture taken at 'The Science Behind Pixar Exhibition at the Telus World of Science in Edmonton'</ref>]]
A physical model of the Cornell box is constructed and photographed using calibrated equipment. [[#Photographic images|Photographic image references]] shared via Cornell University website<ref name="CornellBoxData">[https://www.graphics.cornell.edu/online/box/data.html Cornell Box Data]</ref> were captured using a liquid-cooled Photometrics PXL1300L [[CCD camera]] with a precision of 12 bits. Seven narrow-band [[Photographic filter|filters]] are employed to obtain a coarse sampling across the [[visible spectrum]]. To enhance accuracy, [[Dark current (physics)|dark current]] is subtracted from the images, and [[flat-field correction]] is applied to compensate for [[Vignetting|cosine fall-off and lens fall-off]]. The precise settings of the scene are [[Measurement|measured]], including the [[emission spectrum]] of the [[Light#Light sources|light source]], [[Reflectance#Reflectance|reflectance spectra]] of all [[surface]]s, and the exact position and dimensions of objects, walls, light sources, and the camera.

A matching virtual [[Glossary of computer graphics#3d scene|3D scene]] is created, and a [[digital image]] is generated during the rendering process for comparison with the [[Photo-referencing|reference photograph]]. The comparison helps evaluate the accuracy of rendering algorithms, particularly in handling [[global illumination]], [[Radiosity (computer graphics)|radiosity]], and [[light transport]]. The Cornell box is designed to demonstrate ''[[diffuse reflection|diffuse interreflection]]''. Light reflecting off the red and green walls subtly tints the adjacent white walls, demonstrating complex global illumination effects.

== Scene Configuration ==
The basic environment consists of:
* A single [[light source]]
* A green right wall
* A red left wall
* A white back wall, floor, and ceiling

Objects are commonly placed inside the box to study their interaction with light. The original configuration included two boxes, while subsequent versions introduced a [[Reflection (physics)|reflective]] [[mirror]] [[sphere]] and a [[Refraction|refractive]] [[glass]] sphere, commonly used in [[Ray tracing (graphics)|ray tracing]] research.<ref>{{Cite book|last=Jensen|first=Henrik Wann|title=Realistic Image Synthesis Using Photon Mapping|year=2001|publisher=A. K. Peters|isbn=9781568811475}}</ref>

== History  ==
=== The Original Cornell Box ===
The original Cornell Box was described by Cindy M. Goral, Kenneth E. Torrance, and Donald P. Greenberg in their 1984 paper titled ''Modeling the Interaction of Light Between Diffuse Surfaces'', presented at SIGGRAPH '84.<ref name="CornellBox1984"/>

In this initial version, the interior was painted in red, blue, and gray, and no occluding objects were placed inside the box. Rather than placing a light source inside, the box was illuminated indirectly using a set of lights and a white diffuse surface.<br>
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Cornell_box_render_SIGGRAPH'84.png | The rendered image is on the left, and photo reference on the right.<br>The ray tracing program was written in [[C (programming language)|C]] on a [[VAX-11/780]] [[superminicomputer]].
Cornell_box_setup_1984.png | Sketch of the scene configuration from the same paper with Cornell box referred to as a {{`}}''test cube''{{'}}.
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=== Hemi-Cube Form Factors ===
This simulation of the Cornell Box was carried out by [[Michael F. Cohen]] and Donald P. Greenberg for their 1985 paper ''The Hemi-Cube: A Radiosity Solution for Complex Environments'' presented at SIGGRAPH '85.<ref name="HemiCube">Michael F. Cohen, Donald P. Greenberg. ''[https://dl.acm.org/doi/10.1145/325165.325171 The Hemi-Cube: A Radiosity Solution for Complex Environments]''. SIGGRAPH 1985, Vol. 19, No. 3, pp. 31-40.</ref> The hemi-cube technique allowed [[Form factor (radiative transfer)|form factors]] to be calculated using scan conversion algorithms, which were supported by hardware at the time, and made it possible to calculate shadows from occluding objects inside the scene.

This version of the Cornell Box was the first to feature objects placed inside — a short block on the left side, a tall block on the right side, and a light source in the center of the 'ceiling'. This configuration matches the scene data<ref name="CornellBoxData"/> shared by Cornell University discussed in [[#Scene data|Scene data (section)]]. This layout became a standard reference and was widely reproduced, but in many rendered versions of this scene, the arrangement of the blocks is often mirrored.
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=== Spherical Harmonics ===
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| Photographic image reference<ref name="CornellBoxWebsiteComparisonSubpage">[https://www.graphics.cornell.edu/online/box/compare.html Cornell Box Comparison subpage]</ref>
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François X. Sillion, James Arvo, Stephen Westin, and Donald P. Greenberg made significant contributions to global illumination in computer graphics, particularly through the use of [[spherical harmonics]].

In their 1991 paper presented at SIGGRAPH '91, ''A Global Illumination Solution for General Reflectance Distributions'',<ref name="SphericalHarmonics">{{Cite journal|last=Sillion|first=François X.|date=1991|title=A Global Illumination Solution for General Reflectance Distributions|journal=SIGGRAPH '91 Conference Proceedings|pages=187–196|publisher=ACM|location=Las Vegas, United States|url=https://inria.hal.science/inria-00510145/document?}}</ref> they demonstrated a method that extended light transfer simulations to handle complex reflectance properties beyond ideal diffuse or specular surfaces. This approach utilized spherical harmonic decomposition to encode [[bidirectional reflectance distribution function]]s (BRDFs) and directional intensity distributions, allowing for more accurate and efficient rendering of materials with complex reflectance characteristics. In this version of the Cornell box, the blocks are arranged in a flipped configuration, and the tall block features a mirror-like (aluminum) surface instead of a diffuse one.<ref name="SphericalHarmonics"/> This modification does not align with the scene data<ref name="CornellBoxData"/> shared by Cornell University and could contribute to the misconception described in [[#Scene data|Scene data (section)]].

The photographic and synthetic image references found in the SIGGRAPH '91 paper,<ref name="SphericalHarmonics"/> in a specific subpage of the Cornell University website named 'Cornell Box Comparison'<ref name="CornellBoxWebsiteComparisonSubpage"/> and in the officially provided reproducible data<ref name="CornellBoxData"/> are all unique, even though they look fairly identical. Common misconceptions are discussed in the section below, but it is worth noting that while the synthetic image found in this section is entirely different from the 'officially' provided synthetic reference,<ref name="CornellBoxData"/> the photographic reference closely resembles the 'official' photographic image discussed in [[#Photographic images|Photographic images (section)]]. At first glance, it is easy to notice the changed camera position, as well as the addition of a square object obstructing and absorbing light — a so-called '[[Flag (lighting)|flag]]', often used in photography to achieve the opposite effect of '[[Reflector (photography)#Board reflectors|bounce cards]]'. It was likely used to block distracting effects caused by the strong light source being directly visible by the camera. This absorbing object is not present in the officially provided [[#Photographic images|photographic image (section)]].

== Cornell University data and common misconceptions ==
Publicly released resources from Cornell University contain mixed data that has led to some misconceptions. Over the years, edits were made to the website,<ref name="CornellBoxWebsite">[http://www.graphics.cornell.edu/online/box/ The Cornell Box website]</ref> but few details about these changes are provided, and no documentation exists regarding the modifications. The website was created before web archiving services became widely available, meaning some updates may not be accessible or documented.
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=== Synthetic image ===
[[File:Cornell_box_render_by_Cornell_University_Program_of_Computer_Graphics.jpg|thumb|Synthetic image reference contributed by ''[[Cornell University]] Program of Computer Graphics'']]
A rendered image reference was uploaded by the ''Cornell University Program of Computer Graphics''. The camera position and Cornell box configuration match the provided scene data,<ref name="CornellBoxData"/> but short and tall blocks are mirrored horizontally. The synthetic image contains a shading artifact near the left corner of the short block, making it easily recognizable. The website provides no details on the ray tracing or rendering methods used. No publicly available photograph of the original reference exists, aside from the photographic images discussed in the section below. As a result, a rendered image with artifacts, which is over 30-years-old, is still often used as a reference for recreating the scene, despite significant advancements in ray tracing software and hardware. Some examples that can be found on this page were path traced with the unbiased [[Octane Render]] rendering system, but intentional biased inaccuracies had to be introduced to mimic the limitations of older software. Without these adjustments, modern results would be too accurate and would deviate significantly from the available synthetic reference.

It's a bit of a paradox now: this reference image, the Cornell Box itself, and its role in research that helped to pave the way for the advancements seen in modern ray tracers. These very same modern ray tracers must now be deliberately constrained when recreating this image, because they are capable of producing results that are far more accurate and realistic than what was initially possible.

The synthetic Cornell Box image reference was created at a time when rendering technology was still a work-in-progress. The limitations of the source data and the absence of the original non-synthetic photographic image reference are key reasons why the Cornell Box, in its currently available form, is no longer suitable for modern applications. While it doesn't sound like a paradox, it's quite natural that some test tools, assets, or references become outdated over time because they fulfilled their task or newer solutions became available. However, the fact that people recreate an old, already rendered image of an asset — originally created to test and improve things — because the original reference is unavailable does sound like a little paradox. Of course, it was a gesture from Cornell University to share the data used in the research and it's usually used recreationally by people who find it interesting (apparently).

Over the years, advancements in software and hardware have led to significant improvements allowing more accurate reference gathering and measurement. Today, even an average social media profile picture has a higher resolution than the provided synthetic reference. Despite this, the Cornell Box remains an important historical artifact and an enduring icon. In many ways, it is the [["Hello, World!" program|'Hello, World!']] of ray tracing world.

=== Photographic images ===
Cornell University shared data that must be manually processed before being assembled into an image (e.g., through [[multispectral imaging]]). Few details are provided, except for the camera used, the implementation of seven narrow-band filters (to capture multiple wavelengths of the visible spectrum), and applied post-processing (noise reduction and lens correction). The captured wavelengths are briefly mentioned in the [[filename]]s, and seven different [[Monochrome|monochromatic]] sets are available. The specific configuration and capture time of the data are not listed, except for the reference to the data being taken 'in its current configuration' — which refers to the box's configuration at the time of capture, and is fairly self-explanatory. The data has been available on the website since at least 1998.

The 'final' processed image is not available for download, and no official or unofficial renditions have been released. To display it, the original data must first be interpreted and assembled. Because no examples are accessible online, an image was crafted specifically for this article and released on [[Wikimedia Commons]], along with some additional context.

Images found below were carefully recreated using available data, and they are '''for illustrative purposes only'''. Different methods and techniques can be used, leading to a variety of possible results. It is not difficult to notice that the compiled image (the result of digitally processed data) does not match the synthetic image reference<ref group="𝓲">Synthetic image reference provided by Cornell University discussed in [[#Synthetic image|Synthetic image section]]</ref> (e.g., the tall block has a highly specular surface) or the scene data<ref group="𝓲">Reference scene data provided by Cornell University and discussed in [[#Scene data|Scene data section]]</ref> (e.g., the camera position has been offset to reduce a distracting reflection on the tall block).

Seven monochromatic datasets were captured at different visible spectrum wavelengths. Using information obtained from the filenames, we can determine the captured wavelengths: 700&nbsp;nm, 650&nbsp;nm, 600&nbsp;nm, 550&nbsp;nm, 500&nbsp;nm, 450&nbsp;nm, and 400&nbsp;nm. These datasets can be combined to create [[RGB color model|red, green, and blue channels]], which are then used to produce a polychromatic image. One method to create these channels is by assigning the closest corresponding colors on the visible spectrum. The channels were assigned as follows:

* Red channel: 700&nbsp;nm, 650&nbsp;nm, 600&nbsp;nm
* Green channel: 550&nbsp;nm, 500&nbsp;nm
* Blue channel: 450&nbsp;nm, 400&nbsp;nm

Basic corrections were not applied, so no adjustments were made to [[Contrast (vision)|contrast]], [[Colorfulness|saturation]], [[Color balance|white balance]], etc. No advanced corrections (such as lens corrections for [[chromatic aberration]]) were performed. Minimal [[Tone mapping|tonemapping]] was applied, and no pixels were cropped. [[Defective pixel]]s and capture errors were retouched, but the unedited version is also available. The same principles apply to the synthetic renders, which were generated with a slightly offset camera, as the precise data for this particular shot is not provided. The camera was not properly calibrated and is only vaguely aligned with the short block.

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    | [[File:Cornell Box multispectral imaging.png|thumb|center|600px]]
    | Photographic image reference
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    | Unedited version with the capture error visible near the edges of the image.
    | [[File:Cornell Box multispectral imaging extras.png|thumb|center|600px]]
    | Enhanced version with better visibility of construction elements, dust on the reflective surface, and caustics caused by surface irregularities and the sharp edges of the reflective tall block.
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    | Enhanced version with more pronounced defects like bloom, lens flare, chromatic aberration, misalignment between different wavelength captures etc.
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    | Example rendered with [[Octane Render]]
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    | Example showing a distracting reflection when the original camera coordinates are used.
    | [[File:CornellBox OriginalCamera DistractingReflectionGreen.png|thumb|center|600px]]
    | Example showing weaker contrast when the original coordinates of the blocks and camera are used.
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=== Scene data ===
[[File:Cornell Box (flipped) and camera parameters.stl|thumb|left|100px|3D Cornell box model]]
The original geometry data supplied by Cornell University clearly defines the positions of the objects, with the tall block on the right and the short block on the left, as shown in the [[#Hemi-Cube Form Factors|Hemi-Cube Form Factors section]]. While 3D applications and algorithms may interpret axes differently, causing potential confusion, this does not affect the specific misconception in this case. We have direct access to the original data, and although we understand the correct configuration, it's unclear why a mirrored version (flipped along one axis) is more prominently used.

This change was first observed in the '[[#Spherical Harmonics|Spherical Harmonics]]' rendition of the box. Although the authors did not explicitly comment on this change, a plausible explanation could be that without the flipped layout and repositioned camera, the uneven reflection on the tall block would be too distracting. Additionally, if the green wall were visible in the reflection, it would not contrast as strongly as the red wall does on the white background.<ref group="𝓲">Examples can be viewed in [[#Photographic images]] section.</ref> Repositioning the objects and camera could be a more efficient solution than recreating and repainting the entire box.

An accurate method for setting up the camera according to the original specifications is to set the focal length to 35mm and the sensor size (film gate) to 25mm. Alternative setup methods, including the camera's position, rotation, and other relevant details, are provided in the description of the 3D Cornell Box model featured in this section. The alternative coordinates that could be used to reproduce the repositioned camera were never published.

== Historical Context and Applications ==
The Cornell box was developed in the early 1980s as part of research into [[radiosity (computer graphics)|radiosity]], one of the first rendering techniques capable of [[Simulation|simulating]] diffuse interreflection. This work laid the foundation for [[physically based rendering]] methods, played a pivotal role in validating [[global illumination]] and inspired advancements in ray tracing methods.

Later, the Cornell box was adapted for evaluating newer methods such as [[photon mapping]], introduced by [[Henrik Wann Jensen]] in the 1990s,<ref>{{cite web |last1=Tamstorf |first1=Rasmus |last2=Jensen |first2=Henrik Wann |title=Adaptive Sampling and Bias Estimation in Path Tracing |url=http://luthuli.cs.uiuc.edu/~daf/courses/rendering/papers-2/RTHWJ.article.pdf |publisher=Technical University of Denmark|year=1996 |access-date=March 18, 2025}}</ref><ref>{{cite web |last1=Jensen |first1=Henrik Wann |last2=Christensen |first2=Per |title=High Quality Rendering using Ray Tracing and Photon Mapping |work=SIGGRAPH 2007 Course 8 |date=August 5, 2007 |url=https://graphics.pixar.com/library/HQRenderingCourse/paper.pdf |publisher=Pixar Animation Studios |access-date=March 18, 2025}}</ref> which improved the simulation of [[Caustic (optics)|caustics]] and indirect lighting. Modern applications of the Cornell box extend to testing [[Monte Carlo method|Monte Carlo]] [[path tracing]], [[machine learning]]-based rendering techniques and other advanced approaches to rendering. It's also frequently used as a [[Benchmark (computing)|benchmark]] and remains an essential test scene in many rendering [[Software engine|engines]] like [[Blender (software)|Blender]], [[Unreal Engine]], and [[Arnold (software)|Arnold]].<ref>{{Cite book|last=Pharr|first=Matt|title=Physically Based Rendering: From Theory to Implementation|year=2016|publisher=Morgan Kaufmann|isbn=9780123785800}}</ref>

Since its creation in the previous millennium, many companies have developed their own updated and higher-quality references for internal use, as advancements in technology have allowed for more detailed and accurate measurements. Although primarily used for computer graphics, variations of the Cornell box have also been employed in acoustics research to model sound reflections and validate simulation methods.<ref>{{Cite journal|last1=Tsingos|first1=N.|last2=Carlbom|first2=I.|last3=Elko|first3=G.|last4=Kubli|first4=R.|last5=Funkhouser|first5=T.|date=2002-07-01|title=Validating acoustical simulations in the Bell Labs Box|journal=IEEE Computer Graphics and Applications|volume=22|issue=4|pages=28–37|doi=10.1109/MCG.2002.1016696|issn=0272-1716|url=https://hal.inria.fr/inria-00606718/file/cgaFinal.pdf}}</ref>

== See also ==
* [[Utah teapot]]
* [[Stanford bunny]]
* [[Stanford dragon]]
* [[List of common 3D test models]]
* [[Path tracing]]
* [[3D modeling]]

== References ==
<references />

<!-- Annotations declared as a separate group so they are not listed (or confused) with references. Implemented this way to benefit from the "mouse over" (hover) CSS feature and auto numbering. Other features like efn/sup/abbr won't be this efficient or easy to edit by future users. Don't hesitate to change the letter used before the numbering (that fancy 'i'), but if it's not a single character it will break a lot of formatting here :-) -->
== Annotations (𝓲) ==
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== External links ==
{{Commons category|Cornell box}}
* [http://www.graphics.cornell.edu/online/box/ The Cornell Box website]

{{Standard test item}}

[[Category:3D graphics models]]
[[Category:Test items]]
[[Category:Cornell University]]