Would you mind posting a mock-up (the selection and the result ou want)?
Brushes cannot be applied to Selections per se.
The Selection can be used to limit the effect of the Brush or as a Layer Mask, one can create a Work Path from the Selection and »Stroke path with brush« (but that would only be an outline), …
The closest to what I suspect you want might (other than manually painting the whole thing) be creating a zigzagging or squiggly Path covering the image and then applying »Stroke path with brush« to that.
You could do it with the Paint Bucket set to pattern
- Select an area with the rectangular Marquee tool on a layer that contains the pixels you want to use. It can be any shape so long as it has square corners
- Edit > Define pattern - and give it a name
- Select the Paint bucket, and from the Options bar use the drop-down to change Foreground to Pattern, and the next drop-down along to select the pattern you just made.
- Open a new layer and click to fill with your pattern.
But if you are trying to fill with stars, you'll be hard pressed to achieve a seamless pattern, but there are tutorials out there. Like This
If you only part of the image covered then you'll need to select it, but I would tend to make the selection and add a layer mask so you can fine tune the boundries.
Thanks, just tried that. Unfortunately it seems that the pattern sample has to be really large and even in order for it not to obviously be a repeated selection... Thats ok, but i'm not very good at using the paint/erase tool to create an even coverage. It tends to end up with obvious strokes, and with the brush set i've downloaded the brush size has to be really small to get the kind of detail I want, so it will take me a long, long time to create an even coat over the entire canvas, or a section big enough not to be noticeable via the above technique...
I'm very much a PS beginner, does anyone have advice on how to paint evenly over large areas without making the strokes obvious? I guess this is probably a photoshop skill that takes practice....?
You don't paint, you dab. Use one of the tutorials I linked to to do a general background dusting of stars, and then load the star cluster brush set, select one, size it, and dab it once on a new layer. Select another star cluster brush, and place that on a new layer. Move and size the layers for best effect. Go search for some planet brushes, and place one or two in the scene.
How about finding some smoke brushes; stamp a nice on your canvas, and clip a Hue/Saturation adjustment layer to it. Click on colorize, and adjust. Instant nebula
Build the scene layer by layer, ajusting opacity and size with Free Transform. If a layer doesn't work, turn it off. You can use the same star brush more than once by flipping and/or rotating it.