Barrel

This plugin corrects Barrel type or Pin Cushion type distortions in an image that result in due to camera lens projecting image on to a flat film or sensor array. ( The distortions due to perspective are not compensated by this plugin. For correcting perspective distortion one need to use Reform function of this plugin.) This plugin can also correct for distortions which are different along x and y directions In the test mode a distorted matrix of dots is overlain on the specified frame

Input formats RGB Y or YUV444 only are acceptable.All bit depths are handled. This function is Thread safe. MT_NICE_FILTER

A polynomial with 4 coefficients are used to correct. a, b, and c are the coefficients to be specified and the fourth d is computed internally. These parameters mainly depend upon the lens of the camera, focal length of lens and setting of zoom.

For many high definition still photographic cameras databases are available on the web. But for video cams there is no such database. The possible reasons are 1. Video camera resolution is limited 2. The camera and or subjects move and therefore such distortions are not noticeable. However, due to advent of HD video cams and the consequent increased usage of tripod one may attempt to correct for such distortions.

By the use of test mode the values of coefficients can be arrived at. For a particular camera and setting of focus, these coefficients will remain same. In test mode the number of steps, the range of each of the coefficients etc. are specified. Then a clip with the selected frame of input clip as background is overlain with dots which lie along distorted lines. Each increment of output frame, these coefficients are altered by a step. This display may aid in selecting proper coefficients.

On some images unequal vertical and horizontal distortions are noticeable as in cinemascope. A parameter vh can be used. This specifies the ratio of vertical to horizontal distortion.

>Note. This Barrel or Pin cushion distortion correction must be done on un cropped image or on symmetrically cropped image as it assumes the center of distortion to be the center of frame.

Details of parameters

Description	Name	Type	Limits	Default
Input clip		clip	RGB or planar format only	none
coefficient a (xa in independant mode and start val in test mode)	a	float	between - 0.5 and 0.5	0.005
coefficient b(xb in independant mode and start val in test mode)	b	float	between - 0.5 and 0.5	0.005
coefficient c(xc in independant mode and start val in test mode)	c	float	between - 0.5 and 0.5	0.005
whether to correct pin cushion or barrel type distortion	pin	Boolean	true for pin, false for barrel	false
vertical to horizontal distortion ratio	vh	float	between 0.1 and 10	1.0
whether in test mode	test	Boolean	true for test, false for normal	false
end value of coefficient a in test mode(xea in independant mode)	ea	float	between - 0.5 and 0.5	0.05
end value of coefficient b in test mode(xeb in independant mode)	eb	float	between - 0.5 and 0.5	0.05
end value of coefficient c in test mode(xec in independant mode)	ec	float	between - 0.5 and 0.5	0.05
number of steps for variation ( in test mode only)	nsteps	integer	between 0 and 100	20
input frame number for over lay ( in test mode only)	frame	Int	within input clip	0
color of dots in test mode and left out borders in normal	color	Int	in RRGGBB format	0
whether x and y have independant coefficients	ind	Boolean	true for independant, false for pin cushion or barrel	false
whether Y direction has pin cushion distortion?	ypin	Boolean	true for pin cushion, false for barrel	false
value of coefficient ya ( start value in test mode )	ya	float	between - 0.5 and 0.5	0.05
value of coefficient yb (start value in test mode)	yb	float	between - 0.5 and 0.5	0.05
value of coefficient yc (start value in test mode)	yc	float	between - 0.5 and 0.5	0.05
end value of coefficient ya ( in test mode only)	yea	float	between - 0.5 and 0.5	0.05
end value of coefficient yb ( in test mode only)	yeb	float	between - 0.5 and 0.5	0.05
end value of coefficient yc ( in test mode only)	yec	float	between - 0.5 and 0.5	0.05

#Usage examples:-

blankclip(width = 720, height = 480, pixel_type = "RGB32",color = $ffffff,  length = 10)
grid()
pin = Barrel(a = 0.005, b = 0.005, c = 0.005,pin = false)
 bar = Barrel(a = 0.005, b = 0.005, c = 0.005,pin = true)
 test for image with independant distortions:-
 Barrel(test=true,pin=false,  ind = true,a= 0.05,b= 0.011,c= 0.001, ea = .02, eb=0.012,ec= 0.001, ya= 0.1, yb= 0.011,yc= 0.001, yea= 0.01, yeb= 0.005, yec = 0.005 , ypin = true, color = $ffffff)

Barrel Distortion Example:-

Pin Cushion Distortion Example

Pin cushion and Barrel simultaneous. Use ind = true to correct

with following script dots displayed
Barrel(im,a = 0.005, b = 0.005, c = 0.005,pin = false, test = true, color = $ffffff)

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