In here I pickup a pre-made texture and try and improve it. I'll explain what considerations I make and how I deal with different situations.
This is the texture (PSD). It was half-arsed, I admit, I didn't take it very far and the metling areas on the the top were not blended-in well. Let's try and fix that and perhaps add some stuff!
First, I start from the bottom, actually. I want to make the lower area more unique, more dirty. It looks like it should be darker, but isn't, so I use a Hardlight layer with a dull warmish color to paint the dark areas. Hardlight works as a building-up overlay: it can build-up opacity upon itself with repeated strokes, but initially it is transparent, therefore retains the texture underneath.For special Hardlight tut, click here.
The lower the darker, in this instance. The cracks (or "explosed" areas) get additional rendering in the form of an edge outline (~pixels dark line below the wall and a soft drop-shadow.
This created some "island" floating in the lower areas, places where the wall has been broken but not removed. I like it, and using the Stamp tool on the original layer I add some more island to the area on the right, sourcing from anywhere lighter in the image. These new island should also get some edge/drop shadow definition, just a hint is enough.
With the lower area completed, I move upwards, adding soft lights and darks in-between the texture details, and coloring inside them. Starting from Cyan on the left area --> blue, purple --> yellow, greenish --> connect with cyan.
IIt's time to work the area on the top. The moldy stuff doesn't blend well and doesn't deliver a strong enough visual impact to be considered a valid design element. It needs to be stronger, balder, to compliment the bottom cracks. I decide to make it hanging growth.
First, I create a selection of the target area. Starting from a center overhang in the midde, I add 2 more in the quarters, the left one being somewhat offset for randomness.
Now I prepare my sources. I open a couple of mossy textures I have lying around, the first is brand new and the second is one I've used before, already tweaked. In order to make the former match the latter, I do 2 steps:
It is also vital to match the pixel density of both source and target textures. This usually requires sizing down the high-res photos to an even level.
Finally we can use the Stamp tool to source the growth into selected area: I use the 2nd texture to fill it. Since we used the Lasso tool, the shape is hard-edged and rough. We will fix this next.
Blending it in
In order to make our new element function harmoniously with the stone, there are 3 main factors to consider:
Hue & Saturation
The center growth is too green and satured, pretty easy to fix with a feathered selection and Ctrl-U.
Value & Lighting
Some rendering using a Hardlight brush, scooping a dark, dull and warm swatch from the image. Also add some small red touches for balance. Example in zoom view:
Our Lasso selection is hard-edged and needs to be smoothed out. Unfortunately, the GIF to the right doesn't include this step, but the immediate solution is to blur it:
Select the growth (Ctrl-click the layer), Feather the selection (Ctrl-Clt-D) and - in a new layer underneath - Stamp from the 1st photosource. This will add a fading-out gradient and smooth out the hard edges. However, this also creates an unrealistic drop-shadow effect all around the growth rather than just below it (for a top lightsource). So let's nudge the selection down a bit, Invert it(Ctrl-Shift-I) and delete what's not directly below our (feathered) growth. I merge (Ctrl-E) the 2 layers and duplicate (Ctrl-J) the result for a fuller effect.
Now we go in and paint growth along the edge with small brushes to break the uniformaty of the above effect. Paint consistently with what you see. You might want to zoom into the PSD file to check things up-close. Also I suggest keeping a few "raw" edges for readability.
Photosource attack #2
At this point I look at the image and feel it still needs some blending, like a soft gradient that would blend the growth in Just a gradient would look too plain, though, let's try and incorporate a second texture effect.
I pick a random texture from the mossy folder:
This texture shows great potential: clear shapes seperated - most prominantly- by color. So, let's handle the green stuff by zooming in, using Select Color Range on ~ 80 fuzzyness to pickup the greens. Too much fuzzyness will select in expanse of the earthy areas, so mind that. Duplicate (Ctrl-J) the selection to another layer and we've got the 2 elements seperated and ready for torture: green stuff goes desat and more yellow, earthy stuff more magenta and saturated. You can try anything you like.
Finally, Copy-Merged that onto our image and set on Difference. Reduce the opacity to ~20% for a coherent view and...
No good, too busy. Invert it? even worse. Change colors? interesting, but doesn't help. What to do???
If brute force doesn't work -- use more force!
What I mean is - do not despair! just keep pushing.
So, the effect is overwhelming, it's too much. The logical solution would be to try and remove some of it. Let's use the element seperation obtained from the previous process to remove some of the earthy areas.
Transfer the grass on top of the Difference layer, just to acquire its selection (Ctrl-click layer), and delete it. Feather (Ctrl-Alt-D) the selection, invert it (Ctrl-Shift-I) and using a big soft brush Erase some areas of the earthy stuff. You can also duplicate certain areas to make them more opaque. You can blend things smoothly or roughly, just play with it until you like what you see.
Note that the images on the right show the Difference layer in Normal mode. When manipulating it, you should have it on Difference.
Once the effect has been moderated, it's time to tweak the color with Ctrl-U. Take the hue slider upwards (result = more red) and lightness slider downwards (result = lighter). By selecting the growth layer you can optimize the effect on that area as well.
The Difference effect usually reverses whatever tweak you do, so try different settings until you are satisfied.
At this point the effect is properly blended.