Bend the image plane
Curvature moves output pixels through a radial remap around the selected optical center. Positive values create barrel distortion; negative values create pincushion distortion. Bilinear sampling blends the 4 nearest source pixels to avoid blocky steps.
Zoom changes how much of the remapped source fills the canvas. Center X and Center Y move the distortion focus, which suits off-center subjects or asymmetrical abstract warps.
Why lenses bend lines
Real distortion comes from magnification changing across the lens field: wide-angle designs magnify the center more than the edges, bowing lines outward, while telephoto designs do the reverse. The curvature slider reproduces that radial magnification curve, which is why straight architecture and horizon lines reveal the setting most clearly while organic subjects hide it.
The same mechanism runs both directions, so the slider corrects real lens distortion as readily as it adds fake distortion.
Example: straightening a wide-angle interior
A room shot on a phone’s wide lens often bows door frames outward. Set Curvature to a small negative value, watching one vertical edge near the side of the frame, and stop the moment it runs straight; interiors rarely need more than -15. Raise Zoom slightly to hide the thin unmapped sliver at the edges, and keep the center at 50/50 since the phone’s lens axis sat mid-frame.
For the opposite job, stylized album art or a skate-video look, Curvature at 70 with Zoom near 90 gives an aggressive fisheye bow.
Outside pixels
Strong pincushion settings can request source coordinates beyond the image. Transparent mode leaves those areas empty, and the export switches to PNG so the emptiness survives. Stretch edge clamps coordinates to the nearest border pixel, filling the canvas at the cost of elongated edge detail. Prefer Transparent when the result will be layered elsewhere, and Stretch edge when the file must stay a flat JPEG.