resize-for-banner

Rescale an image into social banner versions using ImageMagick. The image is resized to fit and the leftover space is padded black — never cropped. Use when the user wants a banner, cover photo, or resized version for a social profile or article header. Never overwrites the original — creates new files beside it. Defaults to Twitter/X (5:2, 1500x600); other platforms available on request.

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

Platform specs

Platform	Ratio	Dimensions	Notes
Twitter / X	5:2	1500x600	Header photo. Default.
LinkedIn article	16:9	1200x675	Article header. Usually unnecessary — generated infographics are already 16:9.
YouTube channel	16:9	2560x1440	Channel art; safe area is 1546x423 center.
Blog / generic	any	any WxH	Custom dimensions on request.

If the user does not specify a platform, default to Twitter (1500x600).

Approval gate

• Never touch the original image. All output goes to new files named after the original (suffix added, base name unchanged).
• Confirm the plan with the user before converting: the source image, the target size(s), and the exact output filenames.
• If the user asks for a size the spec does not cover, use that size with the same resize + pad technique and say which platform it suits.

Inputs

• source — Path to the original image, e.g. ai-thoughts/imgs/260806-1656.png. Required.
• sizes — Which banners to produce. Default: Twitter 1500x600. User may request any platform from the specs table or custom dimensions.

Outputs

• imgs/<base>-banner-<platform>.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, target size(s), output filename(s). Get a go-ahead before writing files.

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 "<W>x<H>" -background black -gravity center \
     -extent <W>x<H> imgs/<base>-banner-<platform>.png
   

   • -resize "WxH" (no bang) scales to fit inside the box, preserving the aspect ratio — the image is never distorted or cropped.
   • -extent WxH then pads the leftover space; -background black makes the padding dark and -gravity center centers the image.
   • If the source is taller/narrower than the target ratio, padding appears on the left and right; if it is wider, padding appears on top and bottom.
   • Padding color is black by default; ask the user if they want a different background.

4. Verify every output with identify — dimensions must match the target size exactly.

5. Report the output paths and confirm the original is untouched (git status shows the new files 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.
• Do not invent dimensions. Use the platform specs table or the user's explicit dimensions.

Verification

• identify reports exactly the target dimensions for each output.
• The source image inside the banner is undistorted (aspect ratio preserved).
• The original file is byte-identical (unchanged identify output / git status).
• Output files sit beside the original in the same directory.

Error Handling

• Source not found: list candidate images in imgs/ and ask which to use.
• Output already exists: stop and ask whether to overwrite, or choose a different suffix.
• No ImageMagick: magick is always installed on the user's machines — skip checking for it and run the commands directly.