Updated October 2026
This site's homepage mentions that slides "line up edge-to-edge" — this page walks through exactly how that's computed, step by step, using the same logic that runs in your browser when you upload an image.
Given your chosen preset (say, Portrait at 1080×1350 per slide) and slide count N, the tool first computes the size of one giant virtual canvas: width = 1080 × N, height = 1350. Every slide will eventually be one equal-width column cut from this single composite — this is the key idea that makes edges line up: all slides are cropped from one continuous image, not cropped independently.
Your uploaded image almost never matches the composite canvas's exact aspect ratio, so the tool computes a center crop that fills the entire canvas with no empty space — the same logic as the CSS background-size: cover property. It scales your image up or down (preserving aspect ratio) until it's at least as large as the canvas in both dimensions, then crops evenly from the center on whichever axis has extra.
Once the source is scaled and cropped to exactly cover the full composite canvas, that canvas is divided into N vertical strips of exactly equal width: segment width = (cropped source width) / N. Each strip becomes one output slide, rendered onto its own canvas element at the preset's per-slide pixel size.
Because every slide is a precisely calculated, contiguous slice of the same scaled-and-cropped composite image — not independently cropped or resized — slide 1's rightmost pixel column is adjacent, in the original scaled image, to slide 2's leftmost pixel column, with zero gap or overlap between them. This is what makes consecutive slides line up exactly when swiped in order on Instagram.
When you select "Fit Original," there's no cropping step — the composite height is instead derived purely from your source image's own aspect ratio applied to the full composite width, so the entire source image is preserved with no cropped pixels. The slicing step (step 3) works identically either way.