Frequency Separation - this is how to do it in CSP
There is a Photoshop technique called Frequency Separation used mainly to retouch photographs without losing skin texture. But for artists it could be used in any circumstance where you want to work on an image without losing its texture... such as if you took a photo of paper or canvas art you had done and wanted to then work on it digitally without losing the original texture. PS has a special "apply Image" function for this , But I worked out how to pull off the same technique in CSP... This is what you do ( oh and you could make an Action of this)...
Make a copy layer of your image and then Gaussian blur it ...but just enough to just lose the texture you want to keep. Call it "Blur Copy"
Now change the blend mode of this layer to "Subtract" .... it will all look very dark... don't worry that's what should happen.
3)Make a new layer of this dark image by going to menu Layer>Combine copies of displayed layer. Call it " Texture" Set this new layers blend mode to "Add". and then make a duplicate of this Texture layer and then merge it down with the first texture layer ( so that we are sort of doubling the effect of this texture)
4)Now go to the "Blur Copy" layer and change its blend mode back to Normal
The combination of the "added" texture layer and the Blur layer should look almost identical to the original image. You can then work on the Blur layer ( it does not have to be blurry stuff .. you can make sharp strokes too) and the Texture layer will continue to apply the original texture.
Of course you can also use the technique to re-touch photographs or if you apply the texture to the original ( non blurred ) image you can greatly enhance the original texture in the image.
Hope this is useful
I should point out that if the blend mode of the texture layer changes back to Normal ( when combined with its duplicate) then you will have to reset it back to "ADD"
I have discovered an additional use for this technique .... seamlessly tiled textures. Anyone who has created textures from photographs ( their own or downloaded) will be familiar with the way even tiny fluctuations in lighting ( top to bottom or side to side) results in an unpleasant tiling effect that makes the tiled texture unusable. . I found that you can use this "Frequency Separation" technique to to solve that problem.
A) Run the technique as I described on your original image.
B) hide all layers except the texture layer and a paper layer ... Set the paper layer colour to a mid grey.
C) Create an image of the texture you now see using Layer>Combine Copies of Displayed Layer .......
And then you can create a texture from this layer and it will not have that brightness shift at the edges.
Of course if it is a big pattern you will still get structural tiling.... but I also have a solution for that ( which I provide with my last set of brushes)..
So by using a combination of this technique and my "organic tiling" Action ... I have now been able to produce a wide range of tiled paper and brush textures that tile really well. In fact I have both functions set up as Actions so I just import the picture and run the two actions and its done. If you are interested in creating textures I really recommend it.