A quick guide to creating random fills using the random fill script in Photoshop fill command tutorial
The key thing, I have found with the random fill pattern tool in Photoshop, is that it works best when you pre-size the pattern tile according the document you wish to fill. If you have a document that is a 1000 x 1000 and then fill with a 700 x 700 tiles then the result will not be very spectacular. If you are using a 1000 x 1000 sized document then use a pattern tile around 200 x 200 say (or 200 x 100). Another quick note, make certain there is some transparency to get the best results as if you use just an image as the pattern tile (say filled with a background of a solid color) then you will just end up with a document of colorful pattern stamps in a square shape and at different angles etc (which is fine if that is the requirement from the random fill pattern.) You need a selection of source tiles or stamps to use and you can quickly create your own or use the default pattern tiles supplied with Photoshop and there are many different sets available on the web or patterns such as abstract and tribal decorative pattern tiles and stunning colorful cross / plus pattern seamless tiles for Photoshop, PSP etc
Where is the random fill pattern in Photoshop, you can find it in the edit menu and fill and custom pattern and script option on and the script 'random fill'. There are other options in the script list and they are covered in other tutorials
Personally, I prefer to use the random fill script onto a layer. You can always just delete the layer if it doesn't work out and another feature of the layer is the transparency and how the random fill pattern only fills part of the layer and leaves the lower layers still visible and they can then be manipulated further using tools such as camera raw filter effect in Photoshop as well as adjustment layers and Photoshop styles and you can always use opacity and blending modes to blend the random filled patterns. Of course, you can run the random fill pattern command and just apply to the background layer and just layer more and more patterns and pattern stamps to that layer and that can also be interweaved with effects such as blurs to add interesting variations into the design (or re-colored between fills)
Another good thing about layers is that you can then use transparency from the layer to load a selection and then use styles etc with that or create a mask from the transparency of the layer and the random fill pattern via the layer menu and layer mask and from transparency command which can then be feathered and manipulated in many different ways to create more interesting combinations of results from the fill.
As with all things in Photoshop, the pattern script can be used once or multiple times. You can also combine the result with other effects such as blurs as well as adjustments, you can run the edit menu random fill and use the edit menu fade command with different blending modes etc. The initial panel offers you the option to set the pattern to use (sadly, it is not a sequence of patterns and there are no other options to tweak the tile before the operation so make certain you select the correct tile to use as the next panel does not offer that tile again). You can set the blending mode and opacity and that is that (no scale or rotation etc option there).
Once you finally get to the random fill script panel, you will see an option for adding presets (which may or may not be useful and if all your patterns etc are the same size then perhaps it is quite useful) and you also have a selection of settings to choose from such as
2) minimum scale factor
3) maximum scale factor
4) max distance from path (probably disabled unless you have a path selected)
5) rotate pattern
6) color randomness
7) brightness randomness
Ok, you can increase the density etc and you will see more of the stamps or tiles in the preview. If the image size ratio to the pattern tile size is ok then the preview should match the actual result (as mentioned). Even if you ram the density up to 10 it may never actually entirely fill the entire preview or result with a single random fill pattern application in Photoshop so if you set it to 10 and want it to fill the entire image then run the tool a few times or just increase the size of the tiles by using the minimum and maximum scale factor settings. If you set though scale factor quite low, it will be hard for the density setting to totally fill the entire document unless you add repeat applies.
You can re-size the tiles by setting a minimum and maximum (the tiles are then sized randomly in between those) so if you have a tile of 200 x 200 then the scale factor of 0.1or 10% will result in some fairly small tiles generated (or not depending on the randomization) but if you set the maximum to 3 then you will have a lot of larger tiles and perhaps the quality will not be so great - this is something for you to decide. Changing the scaling does not affect the randomization of the positions but changing the density does (nor does the color or brightness). Sadly, there is no 'randomization' button added to the panel which can be click to change the position etc of the tiles.
You can also rotate the patterns but there is no option to set a mimum or maximum rotation. It is total random rotatation or nothing.
The color randomization is great for changing reds into blues and greens etc at the maxim or for subtle color shifts if set low. Brightness is great for adding some randomization of light and dark artwork into the design. The combination of brightness and color is very good but the result will depend on the quality of the pictures.
The results of the random fill vary depending on the source material. You could use the number '1' as a pattern source in one apply of the fill and then '2' etc in another apply and then 3 etc to build up a very random numerical pattern / background. You could have different images of cats and then select those in turn and fill randomly an image with different cats. Or different letters or different colored shapes etc. It is a pity that the random fill panel doesn't allow for the change of pattern and it is a clunky two pass effect, still all number of combinations of different tile sources can be used to fill the document or selection or frame (the random fill pattern script makes for a great tool to create randomly colorful filled frame artwork)
If you create a very small object such as a 10 x 10 or less circle or square (red, green etc) and then use the minimum settings for the factors and the maximum for the color and brightness and density, you can create some interesting noise effects in Photoshop using the random fill pattern script. May require a few repeat applies to fill the entire image or selection. Always slightly annoying is that the fill has no repeat feature but a very quick workaround is just to record it as an action and then re-apply over and over that way to speed things up
Ok, slightly different but you can use the script with a path. Simply create a path using the pen tool and make certain the path is selected (via the path panel) and then run the edit menu and fill etc and instead of filling the entire document or selection, the pattern follows the line of the path and and as the path can be any kind of design such as a spiral or zizgag artwork etc, you can create some really interesting artworks just by using that feature. If there is no path, the path option in the panel will be disabled
You can find many other tutorials such as Creating beautiful glowing strands designs using the fibers filter in Adobe Photoshop CC 2017 2015 2014 etc and tutorials on how to convert resources from one format to another, how to use Photoshop patterns as a great source for amazing brush strokes tutorial