Resize and converting from `.png` (with transparent background) to `.jpg` adds a black background

[GabrielLyonB]

Hi there 👋 ,

Context

I'm using the image_processing gem to resize_and_pad every image in my website (into different sizes and to make them lighter).
I found that there is an option to add background but it only applies on the remaining area of the resized image. Here is the issue i open in that gem, but their answer brought me here.

My Problem

I have this one case in which, i convert a .png file with transparent background to a .jpg file. The result is a .jpg image with black background.

Questions

1. Is this the expected behaviour?
   1.1. I this is the expected behaviour, is there a way to change the default background?
   1.2 If not, is there a way to add a background to the image?

I recognize that this question might be simply wrong because the way i which that upper level concept ("changing a backgorund") might be different from how is implemented in reality.
Also, i've been trying to read the documentation and the methods descriptions but i can't figgure out which method to use and why.
Apreciate any help.

[jcupitt]

Hello @GabrielLyonB,

The JPG saver will flatten out any alpha before writing pixels, and by default it flattens against black. You can set another colour, eg.:

image.write_to_file "x.jpg", background: 255

Or maybe:

image.write_to_file "x.jpg", background: [255, 0, 0]

For a red background.

[GabrielLyonB]

Thank you @jcupitt for the quick awnser. With this i can add the background options to the saver method in the image_processing library to solve my problem.
The background is only added to those images that do not have a previous background set right?

[jcupitt]

It's just used to flatten out alpha, if that's what you mean.

You can also call flatten yourself, of course.