What happened?

Description

When applying a letterbox transform to a greyscale source image, the letterbox is coming out transparent, when it should be white.

Steps to reproduce

1. Create a new image transform with:

• Mode: letterbox
• Fill colour: #ffffff
• Image format: webp

2. Using this transform, output a greyscale source image and a colour source image. Couple of example images I'm using attached.

Expected behavior

Both transformed images have a white letterbox.

Actual behavior

The greyscale source image has a transparent letterbox.
The colour source image has a white letterbox.

Thanks for taking a look!

Craft CMS version

5.6.11

PHP version

8.2.22

Operating system and version

Linux 6.12.5-linuxkit (DDEV)

Database type and version

MySQL 8.0.36

Image driver and version

Imagick 3.7.0 (ImageMagick 6.9.11-60)

Installed plugins and versions

• CKEditor 4.6.0
• CP Field Inspect 2.0.4