lovell/sharp

Multiply alpha channel (was: Imagemagick's dissolve option)

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#618 opened on Nov 2, 2016

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 (14 comments) (0 reactions) (0 assignees)JavaScript (1,389 forks)batch import
enhancementhelp wanted

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Description

Hi @lovell, Does sharp support the option to dissolve a transparent image? I was looking for something similar to imagemagick's dissolve option. I took a look at the docs but could not find anything. What i wanna achieve is to overlay a watermark, which works well but i would also like to dissolve it by ~35%.

return sharp(watermark)
    .resize(width)
    .toBuffer()
    .then((outputBuffer) => {
      return stream.overlayWith(outputBuffer);
    });

stream is the streamed base image and watermark is the watermark png file which i would like to dissolve.

Contributor guide

Research direction
Investigate how sharp handles overlay compositing and alpha blending. Look into libvips's composite operations and add a multiply alpha (dissolve) option to the overlayWith method.
Tech stack
javascriptnodejs
Domain
backend
Issue type
Feature
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
2
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-2 days
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
JavaScriptNode.js
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
65