Stacker manual

Crop

Once the images are aligned, their edges no longer line up: each has been shifted, scaled or turned a little, and a border around the frame is covered by some images but not others. Crop trims that border and bakes the alignment into every image, so each engine downstream works on identical pixels.

The step is short. The program draws the box for you when the alignment lands, and for almost every stack the only thing to do here is to look at it and press Crop all images.

Crop needs an alignment. Until one is in, the step is dark on the ribbon and the viewer says “Nothing to crop yet” and “Align the images first, then adjust the crop on the reference image.” If you go to Stack without cropping, Stack crops to the automatic rectangle on its own; the step is here so you can see the box, make it smaller, and keep the uncropped images for a second try.

The box

The crop box is drawn on the reference image, with the registered images shown behind it. Everything outside the box is dimmed; the box has a white outline and a white dot at each corner.

• Drag inside the box to move it. It stops at the edges of the image.
• Drag a corner dot to resize it. The opposite corner stays put. A box can be no smaller than 16 pixels on a side.
• Dragging outside the box does nothing, so a missed grab cannot move it.

Straighten, Aspect and Grid

Under Auto rect sit three controls for the box’s shape beyond its corners. All three are live while the box is open and read-only once the crop is baked.

Straighten turns every image about the centre of the reference before the crop, in tenths of a degree, clockwise positive, up to 45° either way. A tilted horizon or a rail that was not level comes straight. The turn is composed onto the alignment: the picture in the viewer turns with the slider, the automatic rectangle is measured on the turned images so the box never reaches past them, and Crop all images bakes the turn into the pixels along with the warp. The alignment the solver found is not changed, so Undo crop puts things back exactly. Reset returns to zero.

Aspect snaps the box to a ratio and holds it while you drag a corner: Free (the default), Original (the camera’s own), 1:1, 4:5, 5:4, 4:3, 3:2, 2:3 or 16:9. Choosing one fits the largest box of that shape centred inside the current box; Auto rect then gives the largest such box inside the automatic rectangle. While a ratio is set a corner drag keeps the shape, anchored on the opposite corner, and stops at the edge of the image. The choice is remembered across sessions.

Grid draws the rule of thirds over the box, two lines each way, for placing the subject. It is on by default and remembered.

Auto rect sets the box to the largest rectangle every aligned image covers, so no edge shows another image’s border. Hover it: “Largest rect covered by every aligned image”. The program sets the box to this rectangle itself when the alignment lands, so the button is for getting it back after you have changed the box by hand.

That is the right box for almost every stack. Make it smaller only to cut something you do not want in the picture, or to stack a single subject out of a wide frame. Do not make it larger: any part of the box that an image does not cover would be filled with that image’s edge, and an engine would read the edge as detail.

Why the automatic rectangle. Every aligned image covers a slightly different part of the reference frame. The automatic rectangle is the part all of them cover. Cropping there, and baking the warp into the pixels at the same time, leaves a stack where no image has a border it had to invent, and where a second Align run would find nothing left to move.

Size and origin

Three facts under the buttons, with the crop’s status under them.

The status line under them reads “Align first — the crop is defined on aligned images.” before an alignment, “Cropping bakes the warp too, so every engine downstream works on identical pixels.” when the box is ready to bake, and “Crop already baked into the images.” afterwards. After a bake the three rows keep the box that was baked, so you can see what was taken.

Folded, the section’s caption shows the box’s size, a tick once it is baked, and whether Undo crop has something to put back.

Crop all images

Crop all images bakes the box and the alignment warp into every image. Each image is warped onto the reference and cut to the box in one pass, with the resampling kernel chosen under Stack › Shared, and written back in place of the aligned-but-unwarped image. If Local residual field at Crop is on in Align › Options, that correction is solved and applied here as well, which makes the crop take longer. From here on the stack is a set of same-sized, registered images, and Stack can run.

The button is available when an alignment is in, the crop is not yet baked, there is a box, and nothing else is running. While it runs the step card shows the progress, with Pause and Stop. Hover the button for what the bake will keep on disk for Undo crop; the next section explains that line.

When it lands, the status bar reads “Crop baked into all images”, the step card reads “Cropped {w}×{h} → {w}×{h}” with how many pixels were trimmed on each side, and the Stack step is ready. Any stack results you had are dropped, because the pixels every engine would see have changed.

Why bake? Warping every image once, here, means every engine sees exactly the same pixels, so their results can be compared fairly and a stack can be reproduced later. It is also faster: an engine that reads the stack several times would otherwise warp every image again on every pass. Without the crop, Stack bakes to the automatic rectangle on its own before it runs; if the aligned images share no common area at all, which only happens when an alignment went badly wrong, Stack stops with “Can’t stack these images” instead of stacking garbage.

Changes you make after the bake behave as the Welcome chapter said: re-developing the images under Images › Camera develop keeps the crop, and so does changing an engine. Changing the stack itself, by excluding an image, reordering, or capturing another frame, puts the uncropped images back on its own, so that the stack is never a mix of cropped and full-size frames, and asks for a new alignment.

Undo crop and Keep crop

A crop is the one step that overwrites the images on disk, so it keeps a way back. When you press Crop all images, the program keeps the uncropped images on disk until the next import, or until you press Keep crop. Hover Crop all images before you press it and it says how much disk that is: “Cropping keeps the uncropped images so the crop can be undone: about {size} of disk until the next import, or until Keep crop. It doesn’t slow the crop down. If you never undo crops, turn it off in Settings › Performance.” The size is about the size of the whole developed stack.

Under the buttons a line reports what is kept: “{size} of uncropped images kept for Undo crop; Keep crop frees it”. The same line, and the two buttons, appear on the Crop step card at the top of the window while the crop is baked, so they are at hand from any step.

Undo crop puts the uncropped images back and reopens the crop box where it was, with the alignment still in. Hover it: “Put the uncropped images back and reopen the crop box; stacks made since are dropped.” Touch-up strokes made since are dropped with them, because they were painted on pixels that no longer exist. The status bar reads “Crop undone — the images are full size again”, and the program returns to the Crop step with the box ready for another bake. Use it to make the box smaller or larger after seeing a result, or to go back to Align with a different Method: Align images on a baked crop does this undo for you first.

Keep crop deletes the uncropped images and frees their disk space, once you are sure of the crop. Hover it: “Delete the uncropped images kept for Undo crop and free their disk space.” The status bar reads “Kept the crop — freed {size}”. After it, Undo crop is dark, and Align images is too, since there are no full-size images left to align; re-import the images to start again.

Both buttons are dark until a crop is baked with its uncropped images kept, and while a job runs.

Two things stop a crop from keeping the uncropped images, and the line under the buttons says which:

• “Undo crop is off (Settings › Performance)”. The safeguard is the option Keep uncropped images for Undo crop under Settings › Performance: “Full-size images stay on disk until the next import, so a crop can be undone.” Hover it for the reasoning: a crop keeps the full-size images on disk, about the size of the whole stack, until the next import, so it can be undone; it never slows the crop down; turn it off on a small drive if you never undo crops. With it off, the hover on Crop all images reads “Undo crop is off, so a crop can’t be reversed. Turn it back on in Settings › Performance.”
• “No Undo crop: it needs {need} and the temp drive has {free} free”. The program skips the safeguard on its own when keeping the images would leave the temporary drive nearly full, so a crop never fills your disk. Free some space on that drive and crop again.

Tip. Leave the safeguard on. The uncropped images cost disk only until the next import, and a crop you can undo is the difference between trying a different box in a minute and importing and aligning the stack again.

Status messages on this step

The status bar reads “Drag inside the rect to move it · drag corners to resize”. At its right the latest message shows “Crop baked into all images” after Crop all images, “Crop undone — the images are full size again” after Undo crop and “Kept the crop — freed {size}” after Keep crop.

The step card reads “Cropped {w}×{h} → {w}×{h}” once a crop is baked, with “Trimmed {l} px left, {t} top, {r} right, {b} bottom” and the line about what Undo crop is keeping under it, and the Undo crop and Keep crop buttons while the uncropped images are kept. When the program has no record of what was trimmed, the card reads “Crop baked into the images”.

A dark step card says “Align the images first — the crop is defined in reference space”. A step card reading “Skipped” means Stack went ahead without a crop; hover it: “Skipped — Stack will crop to the automatic rectangle”.

The Crop section’s own status line reads “Align first — the crop is defined on aligned images.”, “Cropping bakes the warp too, so every engine downstream works on identical pixels.” or “Crop already baked into the images.”, as the section above describes.

The factory shortcut Go to Crop is Cmd Shift 4; the Reference chapter lists it with the others, and Settings › Shortcuts rebinds it.