Instagram grid and carousel
Splitting a photo into a grid for the Instagram feed sounds simple, but a badly computed grid produces tiles of slightly different sizes, and Instagram re-crops each piece again on publish, which throws the visible layout on the profile out of alignment. The correct geometry crops the source image before slicing, so every tile comes out exactly the same size.
1000 is not a multiple of 3, so the centre crop removes the division remainder before slicing.
No pixel is discarded, but the grid becomes "uneven" (the last row and column absorb the remainder).
It is the same grid engine used for 3×3, just with one row, the preset exists because it is the most requested carousel format.
Split an image into 3 columns (and as many rows as you want). Post the tiles in reverse order (bottom to top, right to left) so they line up correctly in the feed, forming the complete image.
Because the "equal tiles" option was off, and in that mode the last row and column absorb the remainder of dividing the image by the grid, instead of the tool cropping that remainder away. Turn "equal tiles" on for tiles that are always identical.
From 1 to 10 on each axis, so the practical limit is 100 tiles on a 10×10 grid, more than enough for any feed mosaic.
The crop is always centred and, in most cases, removes only a few pixels from each edge (the remainder of dividing the dimension by the number of rows or columns), so the visual impact is usually minimal. Photos where the main subject already sits very close to the edge are the ones most affected by this crop.
The 3×3 grid builds a nine-post mosaic that only makes visual sense when published in order on the profile (the classic "puzzle feed"). The 1×3 strip splits the photo into three vertical slices side by side, the format used for a three-post carousel that someone swipes through inside a single post.
Splitting happens entirely in your browser. No image is sent to any server.