lovell/sharp

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

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#618 opened on 2016年11月2日

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enhancementhelp wanted

説明

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.

コントリビューターガイド

技術スタック
javascriptnodejs
領域
backend
Issue 種別
feature
難度新しい貢献者にとっての実装難度です。1 は非常に小さな変更、5 は専門的な作業です。
3
推定時間経験ある貢献者が調査、実装、テスト、pull request 準備にかけるおおよその時間範囲です。
half day
活動状況Issue が今どれだけ取り組みやすいかを示します。新しい、活発、古い、ブロック中、またはメンテナー入力待ちなどです。
stale
明確さ期待される変更、受け入れ条件、次の手順がどれだけ明確かを示します。
mostly clear
前提条件
Node.jssharp basicslibvips concepts
初心者向け度初回貢献者にどれだけ取り組みやすいかを 1-100 で推定したスコアです。
30
調査方針
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.