The idea behind this tutorial and technique: I'm going to use base image on top of which I'm going to paint "ivy"… the secret behind a continuous string of green leafs is that they’re all on one layer. Shall we begin?
Let's start with finding image of the building and an image of randomly placed leafs - ivy or a hedge will be just fine. I've used the following images:
Open (File > Open...) both images and drag the ivy to the base image. Turn on the layer palette (F7) and set up the default brush (F5), for this tutorial use: Scattered Maple Leaves.
Create a new layer on a top of our base image (Layer > New > Layer). Using the selected Scattered Maple Leaves brush, let's draw some basic ivy all around the building. For this one I'm using a crazy pink color to see it better. Done? Let's go to the next step then.
Now let's get into the essence of this tutorial. On the "ivy," layer I'm going to scale down an image of the hedge/ivy using Edit > Transform > Scale… to match the scale of the building. It doesn't have to be super-perfect the first time.
Now mask out (Layer > Layer Mask > Reveal All) the edges of our shrunken image using the same brush as before: Scattered Maple Leaves. Why? Because I'm going to multiply it (Alt-drag) about 30-40 times to fill the whole image with copies of this image - without showing the hard edges of it.
Once copying/multiplying is done, and the canvas is completely filled, Shift-select all those layers and merge them (Layer > Merge Layers).
In this step we're going to hide (mask out) all the "ivy" from the layer we just created in Step 4. Do this by going to Layer > Layer Mask > Hide All. Done? Beautiful!
Now it's time to reveal our "ivy" by Unmasking it in the previously selected areas. We'll do all this using the same brush: Scattered Maple Leaves. Be sure to utilize your Bracket keys to make the brush bigger or smaller while working rapidly. Continue to remove ivy until you're left with a shape that looks good on your building.
In this step I'm going to shift the color of our "ivy" a bit to match the surrounding greens. For this I'm going to use a Hue/Saturation adjustment layer (Layer > New Adjustment Layer > Hue/Saturation) and in Hue/Saturation dialog box I'm going to check: Use Previous Layer to Create Clipping Mask. This way my adjustments will only apply to the layer below "ivy." At this point every one of us is going to that is using a bit different file will need to apply adjustments appropriate to the images you chose.
In this step I'm going to add two more layers to simulate shadows for the "ivy". First let's duplicate our base layer… (Layer > Duplicate Layer).
Now let's desaturate this layer: Image > Adjustments > Desaturate. Using Curves, let's contrast it a bit. Were going to use darker places from this image to create an illusion of shadows (building structure) in the "ivy."
It's almost done. Two more things… put this layer on Multiply - using Blending Mode > Multiply, from your Layers Palette and drag this layer between our "ivy" and Hue/Saturation layer. What's going to happen is that our newly created "shadows" layer is going to appear only where the "ivy" is.
The Previous step could be the last one in this tutorial but there are some additional things we can do to make it look better. As an example, let's copy (Alt-drag) our newly created "shadow layer" and position it between our base image and "ivy." Now we can use this layer to darken places below "ivy" by masking/unmasking it where needed.
This step is totally up to you... if you like colors/contrast of your image - you can leave it as it is or you can experiment with Adjustment Layers to make the colors, contrast, or hue/saturation shift in any direction... as I said: it's up to you!
The last thing is to apply some Sharpening. The "Down-and-dirty" way is to click on a top layer in your PSD file, press: Command + Shift + Alt + E (Merge All Visible Layers) and go to Filter > Sharpen > Unsharp Mask… select the amount and press Enter. Save it and we're done!
Have fun applying the techniques in this tutorial in your next project. The final image is below.