lovell/sharp

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

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#618 ouverte le 2 nov. 2016

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 (14 commentaires) (0 réactions) (0 assignés)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.

Guide contributeur

Stack technique
javascriptnodejs
Domaine
backend
Type d'issue
feature
DifficultéDifficulté estimée pour un nouveau contributeur, de 1 pour un très petit changement à 5 pour un travail expert.
3
Temps estiméFourchette de temps approximative pour investiguer, implémenter, tester et préparer une pull request.
half day
Statut d'activitéDisponibilité apparente de l'issue : fraîche, active, ancienne, bloquée ou en attente d'un mainteneur.
stale
ClartéClarté avec laquelle l'issue explique le changement attendu, les critères d'acceptation et la prochaine étape.
mostly clear
Prérequis
Node.jssharp basicslibvips concepts
Accessibilité débutantScore de 1 à 100 estimant l'accessibilité de cette issue pour un premier contributeur.
30
Direction de recherche
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.