I've spent a fair amount of time investigating Imagine's Depth
of Field feature and thought I'd share my findings. The following text
is long winded, excessively formal and has not been reviewed for errors.
Still, You may find something of use. If not, don't hesitate to
delete...
Depth of Field - Overview:
According to the addendum text file accompanying Imagine 3.0,
the Depth of Field (DOF) feature was added late in development and is
therefore not covered in the manual. While the supplemental DOF text
file is brimming with potentially useful information, it has not been
prepared with the average end user in mind. Compounding this problem is
a less than intuitive DOF interface (DOF and 3D stereo imaging share the
same parameters and requester box). The result is an exceptionally high
learning curve. The goal of this text is to strip away the noise
associated with the mixed function interface and provide a practical
explanation of the Depth of Field feature.
Depth of Field - Useful Photographic Concepts:
Imagine 3.0 simulates many of the real world relationships
associated with photography/videography. These include:
Focal point:
It is the point of perfect focus. Objects nearer or farther
tend to be blurred, although an acceptable field of focus extends for
some distance on the near and far sides of this plane.
Depth of field (DOF):
The Depth of Field is the range of distances from the camera
over which focus is considered adequately sharp.
Field of view (FOV):
FOV is the width of the camera viewing area (usually expressed
in degrees). FOV depends on the lens focal length.
Focal length:
This is the distance behind the lens (usually expressed in
millimeters) where the image will be in sharp focus. The focal length
also provides an indication of the FOV the user can expect from a camera
system. The terms FOV and focal length are used interchangeably in this
text. Common focal lengths and their associated FOV are given below.
focal fov lens
length (degrees) type
(mm)
17 180 fisheye
20 94 wide angle
28 75 wide angle
50 46 standard
55 43 standard
100 24 intermediate telephoto
135 18 intermediate telephoto
200 12 long telephoto
500 5 long telephoto
Aperture:
The aperture of a lens is the opening through which light is
admitted. In photography/videography is desirable to be able to adjust
the amount of light passing through the lens. This is accomplished via
a variable diaphragm. A wide aperture lets in more light but at the
expense of a greatly reduced DOF.
The effect of focal length, aperture size and focal point on DOF:
It is the interaction of the focal length, aperture size and
focal point which determine the DOF for a specific situation. In
general:
a) Shorter focal lengths provide increased DOF;
b) Narrower apertures provide increased DOF;
c) Focal points farther from the camera provide increased
DOF.
Depth of Field - Imagine's Photographic Counterparts:
Imagine's DOF, FOV (or focal length) and aperture controls are
intimately associated with the camera's X and Y sizes.
FOV:
We have all adjusted the camera view interactively via the angle
(A), zoom (Z) and perspective (P) controls associated with the
perspective view in the Stage editor. What follows is a description of
how Imagine numerically handles some of this information.
The camera's FOV depends on the RATIO of the camera X and Y
sizes. These values can be examined via the Transformation requester
(Stage editor) or Camera size timeline (Action editor). The important
thing to remember, is that a particular X or Y size does not determine
FOV, but the ratio of X/Y (X divided by Y) does. There are MANY X and Y
values that will yield the SAME X/Y ratio (and therefore the same FOV).
See the following examples:
X Y X/Y FOV
(degrees)
100 320 .31 18
200 640 .31 18
100 200 .5 55
320 640 .5 55
Changing the FOV numerically:
As the X/Y ratio changes, so does the FOV. If You decrease X or
increase Y (make the ratio smaller) You will zoom in (narrow the FOV).
If You increase X or decrease Y (make ratio larger) You will zoom out
(widen the FOV). When modeling real life situations, the following may
be used as a guide for setting Imagine's FOV numerically.
focal fov X/Y example
length (degrees) ratio X/Y
(mm)
20 94 2.19 1400/640
28 75 0.68 435/640
55 43 0.50 320/640
100 24 0.43 277/640
135 18 0.31 200/640
200 12 0.22 143/640
500 5 0.04 28/640
An examination of the FOV and X/Y ratio values show that they do
not change proportionally (i.e. doubling the ratio does not double the
FOV). The relationship is logarithmic. That is, making a wide FOV
wider requires a much greater change in the X/Y ratio.
Be aware that wide FOVs produce considerable distortion,
especially at the edges (certain portions of the image will appear
disproportionately large).
Focal point:
Imagine's focal point is associated with the camera Y axis (the
camera view points in the direction of the Y axis). Until the DOF
feature is activated (in the Action editor), all objects will be in
perfect focus (infinite DOF) and a specific camera Y size is not
important. When the DOF function is activated the camera Y size will
have a dual role (we already know that the camera size X/Y ratio affects
FOV). When DOF is activated, the camera Y size will also specify the
distance from the camera which is the point of perfect focus (FOCAL
POINT).
Depth of field:
In a real camera, it is the interaction of the lens focal
length, aperture size and the point of focus that determines the DOF. In
Imagine, the DOF can be configured in several ways. It can be set to a
specific numerical value independent of the aperture size or FOV (or
focal length). If desired, the DOF can be tied to an apparent aperture
size and FOV to allow the simulation of camera optics.
Aperture:
Imagine 3.0 has only a limited notion of aperture size. Imagine
uses an apparent aperture size to influence the DOF. This aperture size
does not in any way affect the amount of light reaching Imagine's
virtual camera. Lighting changes must be done by altering the intensity
of ambient lighting or the individual light sources.
Depth of Field - Overview of the DOF/3DS requester:
To activate the depth of field function, You must go to the
Action editor and add an Actor bar to the camera timeline. This will
open up a the 3D Stereo/Depth of Field Requester box. The following
check/data entry boxes deal with the 3D Stereo image features and can be
IGNORED when adding simple depth of field to Your project:
Y Size is (3DS) Screen Distance (check box)
Distance Multiplier (data entry box)
Eye Separation (3DS) (data entry box)
Eye Sep is Screen Width Multiplier (check box)
Eye Sep is Actual Size at Scrn Dist (check box)
The check/data entry boxes that pertain directly to DOF are:
Y size is DOF focus distance (check box)
Aperture size (DOF) (data entry box)
Ap. size is DOF width multiplier (check box)
Ap. size is actual size (check box)
Depth of Field - Activating DOF feature:
Check the "Y size is DOF focus distance" box to activate the DOF
feature.
Depth of Field - Choosing static or dynamic DOF:
Next, check one of the two following boxes:
Ap. size is DOF width multiplier (static DOF)
Ap. size is actual size (dynamic DOF)
Which one You check will depend on whether or not You need to
animate the DOF effect. For example, You may want to animate the DOF
effect to simulate the optics of a real video camera. If You were to
focus a video camera on an object (at some intermediate distance) and
then zoom in (close-up), You would observe that the background would
progressively become out of focus.
Depth of Field - Configuring the Aperture Size (DOF) data entry box:
Imagine will interpret the value that You enter into the
"Aperture Size (DOF)" box differently, depending on whether You chose
the static or dynamic version of the DOF feature.
Static DOF ("Ap. size is DOF width multiplier" box checked):
This DOF option is static in the sense that changes in the FOV
(X/Y ratio) do not alter the DOF. The value You enter in the "Aperture
Size (DOF) box" will be multiplied by the pixel width of the image (set
in the Project editor). The product of which will be the maximum
defocusing that will occur for objects in the distant background (in
pixels). Larger values result in a narrower DOF and greater defocusing
of distant objects. In lieu of using the following equation, entering
a small number (0.01) will usually provide an acceptable starting value.
The equation for determining the Aperture Size (DOF) box value:
A = D/W
where:
A = Aperture Size (DOF) value
D = Desired maximum defocusing (in pixels)
W = Image width (in pixels)
Example: if You wanted the maximum defocusing to be 6.4 pixels
(at distant background) divide 6.4 by the image width (set in
the Project editor):
1) A = 6.4/640
2) A = 0.01
Dynamic DOF (Ap. size is actual size box checked):
When You choose this option, DOF becomes linked to the camera X
size value (see the equation below). This DOF option is dynamic in the
sense that as You change the FOV (X/Y ratio), the DOF also changes. The
dynamic DOF option can be animated to simulate camera optics. Like the
static DOF option, larger values result in a decreased DOF and increased
defocusing of the distant background. In lieu of using the following
equation, entering values in the range of 2 - 4 generally provide an
acceptable starting point.
The equation for determining the Aperture Size (DOF) box value:
A = DX/W
where:
A = Aperture Size (DOF) value
D = Desired maximum defocusing (in pixels)
W = Image width (in pixels)
X = Camera X size
Example: You want a maximum defocusing of distant objects to be
6.4 pixels. You have set the camera FOV. The camera X/Y ratio
is 320/640. The image width (Project editor) is 640 pixels.
1) A = (6.4 x 320)/640
2) A = 3.2
Depth of Field - Setting Point of Focus:
As previously mentioned, once the DOF function is activated, the
camera Y size determines the focal point. The trick is to vary the Y
size (change point of focus) while keeping the desired focal length
(FOV). This is accomplished by first activating the camera line
function (Stage editor/Display menu). Next scale the camera, using the
Y bounding line (Top view) as the guide for determining the point of
focus. As long as the X and Y axis are scaled proportionally, the focal
length (FOV) will remain the same.
Depth of Field - Summary:
This text provides the basics for implementing the Imagine 3.0
Depth of Field feature. Use of 3D stereo imagining (LCD shutter
glasses) with Depth of Field has not be covered. These instructions
should provide a sufficient basis for understanding the Impulse
supplemental text.
While Impulse's implementation of Depth of Field is not
particularly intuitive, it does offer lots of flexibility and artistic
potential. For instance, You could progressively defocus one object
while bringing another into sharp focus (thereby changing the viewer's
point of attention) by transitioning between two equivalent X/Y camera
size ratios. There are undoubtedly many other ways to use this feature
creatively.
Be aware that the amount of defocusing adversely affects
rendering time. Severe defocusing (narrow DOF) not only results in
longer rendering times but also does not always yield photorealistic
results.
Depth of Field - Corrections to DOF text:
In the text I listed focal lengths and their equivalent field of
view. The values are specific to a 35mm SLR type camera system (which I
did not mention). This may mislead some who may want to simulate other
camera systems (video or still). Even though focal length and field of
view are related concepts, I was not technically correct to use the
terms interchangeably.
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