Editing Cornell box (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.
{{essay-like|section|date=March 2025}}

=== 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 [[Cornell box#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 [[Cornell box#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.

<div style="display: flex; justify-content: space-between; width: 100%; gap: 0px; flex-wrap: wrap;">
  <div style="flex: 1; max-width: 900px; border: 1px double #99aecc; padding: 0px;">
    {{Switcher
    | [[File:Cornell Box multispectral imaging.png|thumb|center|600px]]
    | Photographic image reference
    | [[File:Cornell Box multispectral imaging unedited.png|thumb|center|600px]]
    | 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.
    | [[File:Cornell Box multispectral imaging enhanced.png |thumb|center|600px]]
    | Enhanced version with more pronounced defects like bloom, lens flare, chromatic aberration, misalignment between different wavelength captures etc.
    }}
  </div>

  <div style="flex: 1; max-width: 900px; border: 1px double #99aecc; padding: 0px;">
    {{Switcher
    | [[File:Cornell Box3 Octane (6K 8bit).png|thumb|center|600px]]
    | Example rendered with [[Octane Render]]
    | [[File:CornellBox OriginalCamera DistractingReflectionRed.png|thumb|center|600px]]
    | 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.
    }}
  </div>
</div>

=== 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 [[Cornell box#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 '[[Cornell box#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.