resize-for-banner

Resize an image into a Twitter/X header banner (5:2, 1500x600) using ImageMagick. The image is rescaled to fit and the leftover space is padded black — never cropped, so no content is lost. Use when the user wants a Twitter/X banner for an article or infographic. Twitter/X is the only target: LinkedIn and blog headers accept the original image, so never generate a derivative for them. Never overwrites the original — writes a new file beside it.

Resize for Banner (Twitter/X only)

Turn one image into a Twitter/X header banner with ImageMagick, keeping the original file untouched and writing a new file beside it. The image is rescaled to fit and black-padded, never cropped, so no content is lost.

Target — Twitter/X only

Platform	Ratio	Dimensions	Notes
Twitter / X	5:2	1500x600	Header photo. The only supported target.

Not supported, on purpose:

• LinkedIn article headers accept the original image — a 16:9 infographic can be uploaded as-is. A -banner-linkedin.png derivative is never needed.
• Blog / article headers also take the original image. No derivative.
• YouTube channel art (2560x1440 with a safe area) is out of scope for this skill.

Approval gate

• Never touch the original image. Output goes to a new file named after the original (suffix added, base name unchanged).
• Confirm the plan with the user before converting: the source image and the exact output filename.
• Never produce a non-Twitter variant. If the user asks for LinkedIn, a blog header, or "other platforms", say that those accept the original image and stop. Do not invent a size for them.

Inputs

• source — Path to the original image, e.g. ai-thoughts/imgs/260806-1656.png. Required.
• Target — fixed at 1500x600 (Twitter/X 5:2). Not configurable.

Outputs

• imgs/<base>-banner-twitter.png — e.g. 260806-1656-banner-twitter.png.
• <base> keeps the original filename exactly. The original file is never modified.

Procedure

1. Confirm the plan with the user: source image and output filename. Get a go-ahead before writing the file.

2. Check the original dimensions with magick identify "<source>" so you can predict which axis will be padded.

3. Resize to fit, then pad with black with ImageMagick 7 magick:

   sh

   magick <source> -resize "1500x600" -background black -gravity center \
     -extent 1500x600 imgs/<base>-banner-twitter.png
   

   • -resize "1500x600" (no bang) scales to fit inside the box, preserving the aspect ratio — the image is never distorted or cropped.
   • -extent 1500x600 then pads the leftover space; -background black makes the padding dark and -gravity center centers the image.
   • A 16:9 source (most infographics) becomes 1075x600 inside the frame, with about 212 px of black on each side. That is expected — never crop to avoid it.
   • If the source is wider than 5:2, the padding appears on the top and bottom instead.
   • Padding color is black; ask the user if they want a different background.

4. Verify the output with identify — dimensions must be exactly 1500x600.

5. Report the output path and confirm the original is untouched (git status shows the new file as untracked, the original unmodified).

Quality rules

• Never overwrite or rename the source image.
• Never crop the image. Resize to fit and pad — cropping loses content.
• Never generate -banner-linkedin.png, -banner-youtube.png, or any other non-Twitter variant. Those platforms accept the original image. One target only.
• Do not use a size other than 1500x600.

Verification

• identify reports exactly 1500x600.
• The source image inside the banner is undistorted (aspect ratio preserved).
• Black padding appears on the expected axis and the content is centered.
• The original file is byte-identical (unchanged identify output / git status).
• The output sits beside the original in the same directory.
• A re-render matches the existing banner pixel for pixel: magick compare -metric AE <new> <existing> null: prints 0. Do not compare renders with md5sum — ImageMagick writes per-path metadata, so two identical images hash differently.

Error Handling

• Source not found: list candidate images in imgs/ and ask which to use.
• Output already exists: stop and ask whether to overwrite.
• User asks for another platform: explain that LinkedIn and blog headers accept the original image, and that this skill produces Twitter/X only. Do not generate anything.
• No ImageMagick: magick is always installed on the user's machines — skip checking for it and run the commands directly.