jimp-dev/jimp

Bug? Blend mode "SCREEN" does not work on transparent pixels

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#1,019 opened on Jun 5, 2021

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bughelp wanted

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Description

Expected Behavior

I create a new Jimp with all pixels set to 0xFFFFFF00, then composite another image with all pixels set to 0x00000000 over it using Jimp.BLEND_SCREEN, then set the result to opaque, like so:

var pic = new Jimp(25, 25, 0xFFFFFF00).composite(new Jimp(25, 25, 0x00000000), 0, 0, {mode:Jimp.BLEND_SCREEN}).opaque();

This should yield an image with all pixels at 0xFFFFFFFF.

Current Behavior

The result is an image with everything set to 0x000000FF;

Failure Information (for bugs)

It looks like the algorithm simply nulls everything when the destination is at alpha 0? This makes it impossible, for example, to composite a blurred version of a picture that has transparency over itself using screen blending in order to create a halo of light - it will look like a drop shadow instead!

This also happens with ADD, by the way.

Steps to Reproduce

See above for code.

Context

  • Jimp Version: 0.16.1 in a browser
  • Operating System: win
  • Node version: n/a

Contributor guide

Research direction
Investigate the blend mode implementation in Jimp, likely in `src/effects/blend.js` or similar. Focus on the screen blend formula and how it handles alpha channels. The bug indicates that the algorithm nulls the result when destination alpha is zero. Check the current implementation against the correct screen blend formula: result = 255 (255 src) * (255 dst) / 255, and ensure it handles transparent pixels correctly. Also verify if the issue persists with other blend modes.
Tech stack
javascript
Domain
backend
Issue type
Bug
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
Basic understanding of image blendingJavaScriptJimp API
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
40