A general-purpose HTML instruction document for generating a final thumbnail except for vertical cropping. This standard defines geometry and output behavior only.
Guarantee that every generated image is a complete usable thumbnail, already correct horizontally as 16:9, requiring only vertical cropping by height adjustment, with no dependency on top or bottom space.
Generate the image as a COMPLETE 16:9 thumbnail
embedded inside a taller canvas.
ALL important content MUST exist inside a centered
horizontal band that is already 16:9.
The rest of the image (top and bottom) must be fully discardable.Let W = full image width
Define a centered horizontal band such that:
BandWidth = W
BandHeight = W * (9/16)
This band MUST contain the FULL thumbnail.
Top and bottom areas:
- must contain NO important content
- must be safe to remove entirelyThe image MUST remain fully correct if:
- width is unchanged
- height is reduced to match the 16:9 band
No horizontal repositioning allowed.
No manual alignment allowed.Only ONE thumbnail concept per image.
NO:
- grids
- collages
- side-by-side panels
- multiple thumbnails
Everything must form ONE unified image inside the band.Text is optional.
If used:
- must be fully inside the 16:9 band
- must remain readable after crop
- must not touch or depend on edgesReject output if:
- important content exists above or below band
- multiple panels or sections exist
- cropping removes critical elements
- composition relies on full canvas height
- thumbnail is not already complete inside band1. Generate image
2. Identify central 16:9 band
3. Ask:
"If I remove top and bottom right now,
is this already a finished thumbnail?"
4. If NO → regenerate
5. If YES → return imageThe generator is NOT allowed to rely on the user to fix layout.
The image must already be correct BEFORE cropping.The generated image is:
A finished thumbnail, already correct,
simply revealed by cropping vertically.