lovell/sharp

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

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#618 geöffnet am 2. Nov. 2016

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enhancementhelp wanted

Beschreibung

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
SchwierigkeitGeschätzte Implementierungsschwierigkeit für neue Contributors, von 1 für sehr kleine Änderungen bis 5 für Expertenarbeit.
3
Geschätzte ZeitEin grober Zeitrahmen, um zu recherchieren, zu implementieren, zu testen und einen Pull Request vorzubereiten.
half day
AktivitätsstatusWie verfügbar das Issue gerade wirkt: frisch, aktiv, stale, blockiert oder wartet auf Maintainer-Input.
stale
KlarheitWie klar das Issue die erwartete Änderung, Akzeptanzkriterien und den nächsten Schritt erklärt.
mostly clear
Voraussetzungen
Node.jssharp basicslibvips concepts
EinsteigerfreundlichkeitEin Score von 1 bis 100, der schätzt, wie zugänglich dieses Issue für First-Time Contributors ist.
30
Research-Richtung
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.