Reference

What is written down here

The tools on this site turn a picture. These pages explain what the tools are working against — a single number in a metadata block and the long history of programs choosing whether to believe it — and what each kind of turn costs. None of them is a tutorial for a button. They are here because the commonest question about a sideways photograph — why does it look right in one place and wrong in another — has an answer that fits in a few hundred words, and nobody should have to reverse-engineer it from a tool’s output.

• Why a photo turns sideways when you upload it

  Where the disagreement starts: a sensor bolted to a body that turns, a field written instead of a rotation performed, and two programs reading the same bytes to opposite conclusions.

  Reading time about four minutes

• The eight orientation values, one by one

  A drawn glyph for every legal value showing how the samples really sit inside the file, why four of the eight include a reflection, and which two a camera will ever hand you.

  Reading time about six minutes

• Orientation compatibility

  Which destinations read the orientation tag and which ignore it, each row dated and marked tested, documented or untested — and the repair each one needs.

  Reading time a reference table

• 180 degrees or a flip?

  Upside down is two operations. A turn keeps writing readable; a flip mirrors it. Diagrams, a table, and how to tell which a picture needs.

  Reading time about three minutes

• Rotation quality, explained

  Resampling, generations, smooth against hard edges, and what lossless can honestly mean — with a measured table of repeated turns.

  Reading time about five minutes

• Rotate a JPEG without losing quality

  Four turns by re-encoding against four by moving compressed blocks, measured pixel by pixel.

  Reading time about six minutes

• Straightening a horizon without guessing

  Which line ought to be level, how to measure it across the frame, and an exact table of what each degree of correction costs.

  Reading time about five minutes

• Make scans upright for form uploads

  Turn, deskew, keep every edge, then fit the size window — with the measured accuracy of the scan reader and the sourced portal profiles.

  Reading time about six minutes

• Rotate a folder of scans consistently

  Read every tag first, turn each file from its own upright, handle untagged scans, and verify the run with a manifest.

  Reading time about five minutes

What is deliberately not written here

There is no page about choosing a crop ratio, about shrinking a file, or about any sequence of edits on one canvas. Those are different operations with different failure modes, and a reference page that drifted into them would be a reference page written to catch traffic rather than to answer a question. The same discipline governs the tools: the job this site does is making a picture the right way up — a turn, a mirror where the file needs one, and the metadata field that describes both.

The lossless JPEG quarter turn shipped — the compressed blocks are moved and the written file is checked coefficient by coefficient — and so has frame-by-frame rotation of an animated GIF, with each frame’s delay and the loop read back after writing. Perspective correction is still not built, and the pages say so rather than offering half of it.

Where to go after reading

If the second essay told you which value your file carries, the orientation surface is where that reading turns into a repair, and it will print the same value back to you before offering anything. If the problem is a whole folder of scans that all carry the same defect, the batch queue applies the reasoning to every file and accounts for each one. And if the picture is simply crooked rather than mis-tagged, that is a different job entirely and costs a crop, which the essays do not cover because it is geometry rather than metadata.