lovell/sharp

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

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#618 建立於 2016年11月2日

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 (14 留言) (0 反應) (0 負責人)JavaScript (31,994 star) (1,389 fork)batch import
enhancementhelp wanted

描述

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.

貢獻者指南

技術棧
javascriptnodejs
領域
backend
議題類型
feature
難度面向新貢獻者的預計實作難度，1 表示很小改動，5 表示專家級工作。
3
預計時間有經驗貢獻者完成調查、實作、測試並準備 pull request 的粗略時間範圍。
half day
活動狀態議題目前的可參與程度：新鮮、活躍、陳舊、阻塞或等待維護者輸入。
stale
清晰度議題是否清楚說明預期改動、驗收標準和下一步。
mostly clear
前置要求
Node.jssharp basicslibvips concepts
新手友善度1-100 的估計分數，表示該議題對首次貢獻者的友善程度。
30
研究方向
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.