Defish¶

Description

This effect can transform footage shot with a fisheye lens to look like it was shot with a rectilinear lens, and vice versa. It can also be used to straighten the video that was shot with one of these wide angle converters, which are only slightly curvy, or with a semi-fisheye camera, like the GoPro Hero.

Parameters

Parameter	Value	Description
DeFish	Switch	If checked, the transform direction is from fisheye to rectilinear, when not checked, it is rectilinear to fisheye.
Type	Selection	Selects the fisheye angular mapping function used
Scaling	Selection	Select the scaling method
Straighten all edges of video frame	Switch
Interpolator	Selection	Select which interpolation method to use. Options: see below.
Aspect Type	Selection	Select the pixel aspect type[1].
Amount	Integer	Controls the amount of (de)distortion applied to the video. More details below.
Fix camera scaling between 4:3 and 16:9	Integer
Scale Y to affect aspect ratio	Integer
Manual Scale	Integer	Scales the video. Range is 0 to 1000 (divided by 500 to get scaling factor).
Manual Aspect	Integer	Controls directly the pixel aspect ratio. Range is 0 to 1000 mapped to from 0.5 to 2. Only used if Aspect Type is set to manual.

The following selection items are available:

Type

equidistant
orthographic	Straighten all edges of video frame
equiarea	default
stereographic

Scaling

Fit	no cropping but there will be blank area at the borders
Manual	see Manual Scale

Interpolator

Nearest neighbor
Bilinear	default
Bicubic smooth
Bicubic sharp
Spline 4x4
Spline 6x6
Lanczos 16x16

Aspect Type

Square	default
PAL DV	1.067 ratio
NTSC DV	0.889
HDV	1.333
Manual	see Manual Aspect

Parameters explained

Interpolator:: The seven different interpolation methods allow making a quality/speed trade-off. The interpolators are ordered from fast, low quality to (very) slow, high quality. The spline interpolating polynomials are from Helmut Dersch. For real-time use, Nearest neighbor is the fastest because, in fact, it is equal to no interpolation. In most cases Bilinear should be good enough, and on a decent machine should still run in real time. Beyond Bicubic, the quality gain is marginal for a single resampling. Lanczos takes an eternity!
Amount:: Controls the amount of (de)distortion applied to the video. It controls the ratio of fisheye focal length to image half diagonal, but in an nonlinear inverse way, to make the control more „comfortable“. It can be adjusted beyond „reasonable“ values (which differ between the mapping function types), to produce some looney effects. When exploring this range and the image disappears, check the scaling as it could be that the image became too big or too small to see. For some unreasonable values the image might indeed disappear, when there are math overflows or imaginary results (Type Equidistant and Orthographic are more prone to image vanishing). When working in the „special effect“ range, it is always worth to try manual scaling. If the video contains zooming through a curvy wide angle adaptor, the needed amount will vary. In this case use keyframing.

Notes

Tweaking the parameters for best defish

Take a shot of something like a brick wall or bathroom tiles, that has a lot of horizontal and vertical straight lines. Be careful to keep the optical axis as perpendicular as possible to the wall (i.e. keep a maximally symmetrical image in the viewfinder). Use this image to tweak the parameters, primarily amount, type and aspect.

Some examples of effect abuse

These were tried with PAL DV. These examples work best when there is some interesting action near the center of the image.

For a kind of roundish kaleidoscope, try this:

Amount

775

Defish

OFF

Type

equidistant

Scaling

Manual

Manual Scale

300…400

Another crazy distortion:

Amount

921

Defish

OFF

Type

stereographic

Scaling

Manual

Manual Scale

191

For an effect reminiscent of some scenes from the „2001 Space Odyssey“ movie try this:

Amount

900

Defish

ON

Type

Stereographic

Scaling

Fill