Lens Distort TOP

Summary

The Lens Distort TOP applies or removes lens distortion from an image using the Brown-Conrady model. This can be used to rectify images taken from a physical camera, to apply distortion to a rendering for use in augmented reality (AR), or as a visual effect.

The distortion algorithm used by the Lens Distort TOP can be broken down into three components: radial distortion, tangential distortion and the camera matrix.

Radial distortion shifts pixels in the output relative to their distance from the center of the image (defined by the Optical Center parameter). Radial distortion is controlled by the k1, k2 and k3 constants, with k1 having the greatest effect and k3 the least. Positive k values create a barrel or fisheye effect as if the image was wrapped around the surface of a sphere. Negative k values will bend the image inward to create a pincushion or pillow effect. The equations for radial distortion are:

x_distorted = x * (1 + k1 * r^2 + k2 * r^4 + k3 * r^6)

y_distorted = y * (1 + k1 * r^2 + k2 * r^4 + k3 * r^6)

where, 
  r = distance from center

Tangential distortion shifts pixels as if they were on a surface being tilted away from the camera and is controlled by the p1 and p2 parameters. p1 tilts the image up and down depending on the sign, and p2 tilts the image left and right. The equations for tangential distortion are:

x_distorted = x + (2 * p1 * x * y + p2 * (r^2 + 2 * x^2))

y_distorted = y + (p1 * (r^2 + 2 * y^2) + 2 * p2 * x * y)

where, 
  r = distance from center

The camera matrix is defined by the Optical Center and Focal Length parameters and is used to transform the image positions before the distortion is applied. The Optical Center values control where the center of the distortion effect is, while the Focal Length parameters act as a scale on the other effects. The camera matrix is usually represented as a 3x3 matrix:

fx 0  cx
0  fy cy
0  0  1

Distortion values for a camera lens can be determined using OpenCV calibration routines or through functions for a particular camera (see kinectazureTOP class).

• 

  Barrel or Fisheye Radial Distortion, k > 0

• 

  Pillow or Pincushion Radial Distortion, k < 0

• 

  Tangential Distortion, p1 < 0

• 

  Tangential Distortion, p2 < 0

The Layout page has parameters to control how the distorted (or undistorted) image is arranged inside the output image frame. These features can be helpful to preserve parts of the input image that might otherwise be projected offscreen, to preserve input image resolution, or to establish a consistent output frame when lens distortion parameters are changing.

For example, setting the Post Transform parameter to Optimal and keeping Optimal Alpha at 1, the distorted image will be scaled and shifted so that the entire input image is kept within the output frame. Switching the Layout parameter to Native Resolution, will then scale the output resolution so that no detail from the input image is lost. Attaching an Info CHOP will provide Region of Interest (ROI) bounds that can be used to crop out the original distorted image from the output image.

lensdistortTOP_Class

Parameters - LensDistort Page

Invert Distortion invert - Reverse the lens distortion. This will *mostly* undo the distortion applied by another Lens Distort TOP with the same parameters and Inverse disabled. The lens distortion algorithm does not have a precise inverse solution, so an approximate method is used which may not match the original image precisely. The quality of the approximation varies depending on the magnitude of the distortion. Additionally, the approximation aglorithm does not use the k3 constant and any content that was lost outside the bounds of the image cannot be recreated.

K1 k1 - The first radial distortion constant. The k constants move pixels towards or away from the center of the image. Values greater than 0 produce a barrel or fisheye effect, while values less than zero produce a pillow or pincushion effect. k1 has a larger effect than k3.

K2 k2 - The second radial distortion constant. See K1.

P1 p1 - The first tangential distortion constant. Tilts the image up and down.

P2 p2 - The second tangential distortion constant. Tilts the image left or right.

K3 k3 - The third radial distortion constant (not used in the inverse distortion). See K1.

Optical Center center - ⊞ - The position in the image that should be the center of the distortion. Depending on the unit mode selected, the position can be entered as an absolute position measured from the bottom-left corner, or as a relative position measured from the center of the image. The unit mode also controls whether the position is measured in pixels or normalized coordinates.

Optical Center Units centerunit - ⊞ -

Focal Lengths focallength - ⊞ - The focal length components of the camera matrix descibed either as normalized resolution-independent values or as pixels. The focal length acts as a scalar on the other distortion parameters. The normalized unit mode can be used for systems that use millimeters or other physical units. Often abbreviated as Fx and Fy. See OpenCV for details.

Focal Length Units focallengthunit - ⊞ -

Parameters - Layout Page

Layout layout - ⊞ - Determines how the transformed image is arranged inside the final output image space. This value can be useful to preserve the native pixel resolution of the input image, or ensuring a final output resolution and aspect ratio.

Extend Mode extendmode - ⊞ - Determines what values are used when the output image exceeds the bounds of the input image.

Optimal Alpha alpha - Determines how much of the image is preserved when calculating the optimal post transform. When zero, the optimal transform will include only the region of interest. When one, the transform will include all of the pixels from the input image. This value is equivalent to the 'alpha' value used in OpenCVs getOptimalNewCameraMatrix function.

Post Transform transformmode - ⊞ - Choose whether to preform an addition transformation to the image after the lens distortion is applied. This is useful for positioning and scaling the distorted image within the final image frame and can be used to preserve parts of the input image that would be shifted out of frame by the distortion.

New Center newcenter - ⊞ - A new optical center position that allows for shifting the transformed image within the output image frame. Using the same value as the Optical Center parameter will result in no post distortion offset. Using the Optimal Center value will produce the same results as the Optimal transform mode.

New Center Unit newcenterunit - ⊞ - Determines how the new center values are interpreted.

New Focal Lengths newfocallength - ⊞ - A new focal length that allows for scaling the transformed image relative to output image frame. Using the same value as the Focal Lengths parameter will result in no post distortion scaling. Using the Optimal Focal Length value will produce the same results as the Optimal transform mode.

New Focal Length Units newfocallengthunit - ⊞ -

Center Offset centeroffset - ⊞ - Shifts the distorted image within the output image frame.

Center Offset Unit centeroffsetunit - ⊞ -

Scale scale - ⊞ - Performs an additional post-distortion scale on the image. The scale is relative to the original camera matrix, so values greater than 1 will stretch the image, while values less than 1 will shrink it.

Cropping cropmode - ⊞ - Crop the transformed image so that only part of the image will appear in the final output. Unless a custom resolution is given, the cropped region will be used to determine the final output resolution.

Custom Region cropregion - ⊞ - Defines a custom cropping region in the order of Left, Bottom, Right, Top. The values can either be as a fraction of the image (0-1) or as pixels of the input image size. Entering the ROI values from an attached Info CHOP will produce the same results as using the Region of Interest cropping mode.

Crop Unit cropunit - ⊞ - The units used to interpret the values of the custom cropping region. Can be as fractions (0-1) of the image or as pixels of the input image size.

Scale Unit scaleunit - ⊞ - Determines how the scale values are used.

Parameters - Common Page

Output Resolution outputresolution - ⊞ - quickly change the resolution of the TOP's data.

Resolution resolution - ⊞ - Enabled only when the Resolution parameter is set to Custom Resolution. Some Generators like Constant and Ramp do not use inputs and only use this field to determine their size. The drop down menu on the right provides some commonly used resolutions.

Resolution Menu resmenu - A drop-down menu with some commonly used resolutions.

Use Global Res Multiplier resmult - Uses the Global Resolution Multiplier found in Edit>Preferences>TOPs. This multiplies all the TOPs resolutions by the set amount. This is handy when working on computers with different hardware specifications. If a project is designed on a desktop workstation with lots of graphics memory, a user on a laptop with only 64MB VRAM can set the Global Resolution Multiplier to a value of half or quarter so it runs at an acceptable speed. By checking this checkbox on, this TOP is affected by the global multiplier.

Output Aspect outputaspect - ⊞ - Sets the image aspect ratio allowing any textures to be viewed in any size. Watch for unexpected results when compositing TOPs with different aspect ratios. (You can define images with non-square pixels using xres, yres, aspectx, aspecty where xres/yres != aspectx/aspecty.)

Aspect aspect - ⊞ - Use when Output Aspect parameter is set to Custom Aspect.

Aspect Menu armenu - A drop-down menu with some commonly used aspect ratios.

Input Smoothness inputfiltertype - ⊞ - This controls pixel filtering on the input image of the TOP.

Fill Viewer fillmode - ⊞ - Determine how the TOP image is displayed in the viewer.

NOTE:To get an understanding of how TOPs work with images, you will want to set this to Native Resolution as you lay down TOPs when starting out. This will let you see what is actually happening without any automatic viewer resizing.

Viewer Smoothness filtertype - ⊞ - This controls pixel filtering in the viewers.

Passes npasses - Duplicates the operation of the TOP the specified number of times. Making this larger than 1 is essentially the same as taking the output from each pass, and passing it into the first input of the node and repeating the process. Other inputs and parameters remain the same for each pass.

Channel Mask chanmask - Allows you to choose which channels (R, G, B, or A) the TOP will operate on. All channels are selected by default.

Pixel Format format - ⊞ - Format used to store data for each channel in the image (ie. R, G, B, and A). Refer to Pixel Formats for more information.

Operator Inputs

• Input 0: -

Info CHOP Channels

Extra Information for the Lens Distort TOP can be accessed via an Info CHOP.

Specific Lens Distort TOP Info Channels

• optimal_center_x -

• optimal_center_y -

• optimal_focallength_x -

• optimal_focallength_y -

• roi_left -

• roi_bottom -

• roi_right -

• roi_top -

Common TOP Info Channels

• resx - Horizontal resolution of the TOP in pixels.

• resy - Vertical resolution of the TOP in pixels.

• aspectx - Horizontal aspect of the TOP.

• aspecty - Vertical aspect of the TOP.

• depth - Depth of 2D or 3D array if this TOP contains a 2D or 3D texture array.

• gpu_memory_used - Total amount of texture memory used by this TOP.

Common Operator Info Channels

• total_cooks - Number of times the operator has cooked since the process started.

• cook_time - Duration of the last cook in milliseconds.

• cook_frame - Frame number when this operator was last cooked relative to the component timeline.

• cook_abs_frame - Frame number when this operator was last cooked relative to the absolute time.

• cook_start_time - Time in milliseconds at which the operator started cooking in the frame it was cooked.

• cook_end_time - Time in milliseconds at which the operator finished cooking in the frame it was cooked.

• cooked_this_frame - 1 if operator was cooked this frame.

• warnings - Number of warnings in this operator if any.

• errors - Number of errors in this operator if any.

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