How to create an irregular background for an image

[Diesel16]

Hello,  I'd like to create the background of an image as a triangle instead of a rectangle or a square - is this even possible? 

I have created a triptych of images and wanted the background layer to reflect/complement the overall design which is a triangle.

Searching for help on You Tube I saw articles on how tgo remove a background & wondered if perhaps I could make the empty space around my triangle transparent but I do not have enough experience to know how this will look when exported as a file to someone else, as a jpeg for example.

Can you leave a background with transparent pixels? Would it show up as a chequerboard when viewed by someone else or would it pick up a background colour from their computer/viewing medium?

Many thanks for help

[davescm]

You can make a mask and use it to make pixels transparent (where the mask is white the layer is opaque and where it is black the layer is transparent). But to retain transparency you need to export in a format that supports transparency. That means PSD, TIFF or PNG.   JPEG does not support transparency.

Dave

[Diesel16]

Hi Dave
Thank you so much I am very grateful.
It’s also really good to know which formats do & do not support transparency.
cheers

[jane-e]

"Can you leave a background with transparent pixels?"

In addition to Dave's correct answer, the background layer does not support transparency, but you can delete it or make it a regular layer by renaming it (as long as you save in a format that supports transparency.) Background layers have not been required since Alvy Ray Smith came up with the formula αA + (1-α)B and Photoshop 5.5 was released.

https://www.cs.princeton.edu/courses/archive/spring05/cos426/papers/smith95c.pdf

[S_Gans]

In part, your answer depends on where you're going to use the image. But in various forms, .psd, .tif (with an alpha channel in third party programs, like InDesign, Premiere or After Effects), .png, .gif, .bmp will all allow transparency.