coil-kt/coil

Scaled down images look excessively pixelated on desktop

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#2883 opened on Mar 9, 2025

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 (8 comments) (3 reactions) (0 assignees)Kotlin (11,779 stars) (757 forks)batch import
help wanted

Description

Describe the bug

Large AsyncImages scaled down (compared to their original size) look excessively pixelated (on at least) desktop. This doesn't seem to be an issue on Android, so I'm guessing this is not intended behaviour? I can only test these two platforms right now, so it is possible other platforms are also affected.

The effect is most noticeable with screenshots that contain lots of text, as the text becomes completely distorted and unreadable.

To Reproduce

Try to load a high-resolution image with medium-to-small text on it, as the issue is most apparent on images of this type, (e.g. a screenshot of an IDE window) and try to scale it down.

Version v3.1.0

Samples

Original image:

Image

Android scaled down a lot (this is still just about readable!):

Image

Desktop scaled down slightly (already falling apart):

Image

Desktop scaled down a lot (unreadable):

Image

I considered that this is simply due to the higher pixel density of a phone's screen, but the issue is also not reproducible in the emulator so I'm not sure anymore. 😅

Contributor guide

Tech stack
kotlin
Domain
mobiledesktop
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
Basic KotlinCompose Multiplatform basics
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
50
Research direction
Investigate the rendering pipeline for AsyncImage on desktop. Compare the image scaling behavior between Android and Desktop targets. Look into the Coil image loader's configuration for downscaling, specifically the use of `ImageRequest.Builder` and scale options. Check if there is a difference in how the platform's `ImageDecoder` handles large images. The issue may be related to the default `Scale` or `Transformation` applied. Consider testing with explicit `Scale.FIT` or `Scale.FILL` and checking the `size` parameter.