DEPTH GRADING IN 3D CREATION
David Seigle
Depth grading means adjusting the depth of an individual object, a group of objects or the whole scene in a 3D shot. Properly used depth grading can help achieve “perfect 3D”, that is, 3D that conveys a director’s vision with no distracting disparities.
At In-Three we have created a suite of tools (the “In-Three Depth-builder” – IN3D) that allows us to depth grade scenes as part of Dimensionalization®, i.e., creating 3D from 2D content.
Depth grading accomplishes three important functions:
- It provides nuanced artistic control of depth.
- It allows appropriate adjustments to match the requirements of different screen sizes.
- It provides depth control to ensure comfort for the whole family. (To anticipate the explanation of family comfort, a child’s interpupillary distance – which affects perception of 3D – is about 80% of an adult’s. This can be an issue when 3D is created with only adult IPD in mind.)
This discussion describes the areas in which depth grading may be required.
Depth Grading for Artistic Control
Area #1: Depth budget control.
When shooting movies in stereo, generating stereo content using CG or dimensionalizing
2D content, each 3D scene is created with a depth script in mind. The depth script
defines where near and far objects will appear in the final product. Often a director
will want to change these positions when he screens his results. This means reshooting
film, re-rendering CG or depth grading dimensionalized material.
Area #2: Object repositioning and shaping.
Once a director achieves the proper overall depth budget, individual objects or
groups of objects my still need to be adjusted. Perceived height, size, distance
or dramatic impact can be manipulated by depth grading.
Area #3: Control of inter-shot depth continuity.
When a scene cuts from a near focal element to a distant focal element, or vice
versa, it takes time for a viewer’s eyes to adjust to the distance change. This
can be particularly discomforting during fast paced action with short cuts.
Depth grading allows depth to be controlled so that the focal element in one shot can be repositioned to match the depth of the focal element in the next shot. This can change a jarring experience to a comfortable one.
Depth Grading to Adjust for Screen Size
Area #4: Showing 3D content on a television.
When 3D content is created to look proper (i.e., represent a director s vision)
on, say, a 40 foot screen, it will look bland on a 60 inch television. For example
if a 3D shot is depth scripted to show its near objects at 50% of the distance from
the viewer to the screen and its far objects at infinity, that same shot will appear
to range from 89% to the screen to 14% beyond the screen – that is it will be “accordioned
in” from both directions toward the screen and it will lose its dramatic impact.
Depth grading can reestablish the intended impact for TV. In-Three has collected survey data on 3D TV depth preferences and can advise directors on the appropriate depth grading choices for TV content.
Area #5: Showing 3D content on a larger screen.
When 3D content, created to look proper on one size screen, is shown on a larger
one, it quite likely will cause discomforting disparities. For example, a scene
depth graded from 50% to infinity for a 40 foot screen will, on a 60 foot screen,
show the near objects at 40%, that is, closer to the viewer. Distance objects will
appear “beyond infinity”, that is they will cause a viewer s eyes to diverge.
Divergence is one of the most disturbing and nausea inducing artifacts in bad 3D. Depth grading applied to such 3D content is required before it should be shown on larger screens.
Depth Grading for Family Viewing
Area #6: Ensuring comfort for both kids and adults.
A viewer s perception of 3D is related to the interpupillary distance (IPD) of his
eyes. (This relationship is explained in the white paper entitled “Stereoscopic Geometry of 3D Presentations”, at www.In-Three.com.) An adult s IPD averages about
2.5 inches. In children aged 6 to 11, it averages about 2 inches. (See link to
“Anthropomorphic Study of IPD” on In- Three s site.) What this means, for
example, is that any material created to show objects at infinity for an adult will
cause a child s eyes to diverge.
Ensuring there are settled stomachs in the back seat on the way home may require depth grading.
My friend, Lenny Lipton, who knows a lot about these things, tells me that kid s eyes are particularly adaptable and this should not be a worry. However to be safe, In-Three has established recommended depth budget limits to ensure a high level of family comfort and enjoyment.
In each of these six areas depth grading can help directors and producers produce perfect 3D – 3D that will convey one s artistic vision uniformly across all entertainment venues and to all audiences.