Crop
Step: Linear
Cuts margins off the image. You give a number of pixels for each side, and everything outside is removed. Use it to trim stacking edges, to frame a target or to drop a corner with a defect. In Automatic mode the module first removes the borders that hold no data, which aligning and stacking frames leaves behind.
Parameters
| Parameter | Range | Default | Effect |
|---|---|---|---|
| Mode | Manual, Automatic | Manual | Manual crops by the four margins only. Automatic first cuts away the borders without data, then applies the margins to what is left. |
| Top | 0…10000 px | 0 | Rows removed from the top. |
| Bottom | 0…10000 px | 0 | Rows removed from the bottom. |
| Left | 0…10000 px | 0 | Columns removed from the left. |
| Right | 0…10000 px | 0 | Columns removed from the right. |
With all margins at 0 in Manual mode the image stays as it is. If the margins leave nothing of the image, the module shows an error instead of a result.
How it works
The result keeps the pixels inside the rectangle the margins leave. Mono and color images are cropped alike; the file information (exposure time, filter and so on) is kept.
Automatic looks for the largest rectangle without missing pixels. A pixel counts as missing if it has no value in any color channel; this is what alignment leaves at the borders where a frame did not cover the sky. The rectangle is parallel to the image edges and as large as possible, so a notch in one corner costs only as much as it must. If the image has no missing pixels, the rectangle is the whole image. If every pixel is missing, the module shows an error. The four margins are then measured from the edges of that rectangle, not from the image edges.
If the top-left corner moves by an odd number of pixels, the Bayer pattern of an undebayered image moves with it. Siderea updates the pattern, so Debayer still gets the colors right after cropping.
Tips
- Crop after stacking: stacked images are often left with ragged, darker or empty borders.
- Use Automatic with a margin of a few pixels to also remove the noisy rim just inside the data.
- The live preview works on a reduced copy of the image; the margins are scaled to match, so the preview shows the same section that Apply will cut.