jimp-dev/jimp

Grayscale images aren't saved as 8-bit PNGs or JPEGs

Open

#913 opened on Jul 17, 2020

View on GitHub
 (0 comments) (0 reactions) (0 assignees)JavaScript (13,218 stars) (785 forks)batch import
bughelp wanted

Description

Expected Behavior

When writing images to disk after calling greyscale() method should produce 8-bit PNGs and JPEGs.

Current Behavior

Even when greyscale() is called images are still 24 or 32 BPP. After some digging I've discovered that you can get 8-bit PNGs by leveraging the colorType() method. Unfortunately no such thing for JPEGs.

Steps to Reproduce

Jimp.read('image.png').then(image => {
  image.greyscale().write('image_greyscale.png');
});

Jimp.read('image.jpg').then(image => {
  image.greyscale().write('image_greyscale.jpg');
});

Context

  • Jimp Version: 0.14
  • Operating System: Windows 10
  • Node version: 12

Contributor guide

Tech stack
javascriptnodejs
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.
fresh
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
JavaScript knowledgeUnderstanding of image color depth
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
70
Research direction
Investigate the JPEG encoding module in Jimp, likely in packages/jimp/src/image/jpeg.js or similar. Locate where the color type is set during write and ensure that when the image is greyscale, the JPEG encoder uses a single channel (8 bit grayscale) instead of RGB. Check the underlying JPEG library (like jpeg js) for options to set color space. Reference the existing colorType() method for PNG as a guide.