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 (31,994 stars) (1,389 forks)batch import
enhancementhelp wanted

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

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.
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.
mostly clear
Prerequisites
Node.jssharp basicslibvips concepts
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
30
Research direction
Implement a new option in the overlayWith or composite function to multiply the alpha channel of the overlay image by a factor (e.g., dissolve). This requires modifying the C++ binding to apply an alpha multiplier during compositing. Check libvips documentation for VipsComposite or VipsOverlay to see if alpha multiplication is natively supported. If not, implement a custom operation that scales the alpha band before compositing. Add a simple API like .dissolve(0.35) or an option {opacity: 0.35}. Test with various image formats and include unit tests. Look at existing composite implementation in src/operations.cc and libvips source for guidance.