Separate the raster from its intended shape
Stored width and height describe the sample grid. Sample aspect ratio describes the shape assigned to each sample in display. A production file can therefore have a display shape that is different from width divided by height. Check all three values before stretching an owned archive to fit a modern canvas.
FFmpeg documents the relationship as display aspect ratio equal to width divided by height, multiplied by sample aspect ratio. For an invented 720 by 576 raster with a sample ratio of 16:15, the displayed ratio is 4:3. The calculation is a geometry example, not a rule for every file with those dimensions.
Establish the intended presentation
Look for reliable source documentation or a known circular object and other useful visual references. A title card can be misleading if it was already distorted earlier. Preserve the original and distinguish a justified correction from a subjective preference for filling the screen.
Also inspect whether black borders are stored inside the picture. A raster containing a letterbox has two relevant boundaries: the outer encoded frame and the active image within it. Changing only the outer aspect description may leave the active picture wrong. Cropping requires an additional editorial decision and should be reviewed separately.
Choose the destination operation
If the destination respects the source's aspect signalling, preserving the raster and correct metadata may be appropriate. If it requires square-pixel pictures, plan an explicit resampling to the desired canvas. A metadata-only aspect change and a resampling operation do different work; use the one the handoff requires.
Record the input raster, input sample ratio, chosen output raster and output sample ratio. Explain any crop or padding. This makes a later reviewer able to distinguish a shape correction from the removal of picture area, which matters when edge captions or other information occupy the frame.
Review the result in context
Check faces, circles, on-screen text and the edges of the active image in the intended destination. Confirm that an additional player-fit setting has not stretched an otherwise correct file. Review a small candidate before processing a long archive and preserve the ability to return to the source.
The final handoff should state the intended display shape rather than relying on an extension or the word anamorphic. If more than one destination is required, label each derivative by its actual contract. One source can legitimately produce different canvases, provided the treatment is deliberate and traceable.
Questions answered
Does setting sample aspect ratio to one resample the image?
A metadata setting alone does not change the stored raster. Check whether the workflow also scales pixels.
Does a 720 by 576 file always mean 4:3?
No. The intended presentation depends on its sample aspect ratio and any active-picture treatment.
Sources & further reading
Official references for the subjects discussed. Service terms and device software can change.