jimp-dev/jimp

Resizing an image with transparency

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#442 opened on Apr 26, 2018

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 (4 comments) (4 reactions) (0 assignees)JavaScript (13,218 stars) (785 forks)batch import
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Description

Hy guys ! I have a little problem and I can not find answers. When I try to resize a picture with transparency I get a border between transparency and the other color. On white you can see the border really clear on other colors not so much, but is there.

If I use RESIZE_NEAREST_NEIGHBOR the transparent image looks okay, but the quality of other pictures is poor. If I use the other ones I end up with a picture with a border between transparency and color. Do you have any idea ? Any advices ? Thank you !

Original picture: label

Resized picture with RESIZE_BEZIER logo-1083

Contributor guide

Tech stack
javascriptnodejs
Domain
backend
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
half day
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
stale
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
Understanding of image resizing algorithmsFamiliarity with alpha compositingJavaScript knowledgeBasic usage of jimp
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
35
Research direction
Investigate the resizing methods in jimp (likely in src/image/resize.js or similar). The issue arises because interpolation algorithms like Bezier or bilinear blend pixel values without considering the alpha channel, causing a dark/light border. A common fix is to premultiply alpha before resizing and then reverse. Check existing comments for any hints from maintainers. Test with the provided images to confirm the fix.