*Intermediate and above.
This is a semi-automatic masking technique that enables users to mask or knock-out complex and unwanted areas of a picture with ease and accuracy.
Most designs and artworks will require us to do montaging of several images. However, we can only use those images effectively if we are able to extract the “parts” out cleanly.
Step 1
To begin with, we need to extract the model from the background. Duplicate the layer (CTRL+J) as “Layer 1”.
Then, create a “Layer 2” under “Layer 1” and fill it with a bright blue background. This works as a “blue screen” to assist us in identifying areas that are not properly masked.
Model - Image ID: 2625298 © Claude Belanger 123RF.com
Step 2
Double click “Layer 1” to bring out the blending option, and focus your attention on the “Blend If:” option.
You will see a gradient bar with values 0 ~ 255 (with 0 being the darkest point and 255 being the brightest point) and sliders on each end. Selecting the “Blend If: Gray” will allow Photoshop to use Grayscale of the image as a base for blending.
Step 3
By dragging the left slider to the right by 50 points, it will blend or knock out the darkest parts of the image, making it transparent up until the value set, hence showing the backdrop (i.e. blue screen).
Step 4
Likewise, dragging the white point slider will result in the image below, blending or knocking away some of the brightest points of the image. Notice that the blended area are hard-edged and jagged, which we are going to remove in the next step.
Step 5
What we want to achieve now is a smooth transition in blending (with no hard-edges).
Hold down the ALT key to split and drag the slider (until a desirable level of blend is achieved). The inside slider will soften the edge base on how much it is being dragged. Shades of gray between the split sliders will determine the transparency of affected areas.
Step 6
I’ve decided to refine my settings to 90/190 - to gain more transparency around the hair area (as circled).
Step 7
Portion of the face have been blended as well. To mask out, select “Layer 1” and duplicate it (CTRL+J). Double click into the blending option and reset the sliders. Click on the masking tool and fill it with black.
Select a white color brush and slowly brush to reveal the face, skin, or any areas that you do not want to mask. Areas with hints of blue tone should be brushed away to prevent any unwanted "holes" in the object.
Step 8
When you are done, hold CTRL and click on “Layer 1”. Merged them (CTRL+E) and the image is ready to use.
Note: Before merging, be sure that this is the image you want. If you are not satisfied with the masking results, changes or tweaks can still be made by accessing the blending option. Once when you are satisfied, merge the layers toegther (CTRL+E).
Another note: Reveal the blue screen to double check if the background is fully blended away.
An example:
Sunset - Image ID: 476483 © David Mark 123RF.com
When to use, and what to use?
Not all images can be masked using this method. Some white edges could still be visible. It will require further masking to eradicate unwanted edges.
Knowing which method to use is crucial to achieve a desirable result in a timely manner. Images with higher contrast ratio are easier to manipulate with the “Blend If:” function.
To identify which image contains higher contrast ratio, bring out the Channels Window (Windows > Channel).
In the image below, take Blue channel for instance. The brighter/whiter the area, the more blue it contains. Thus, out of the 3 channels below, blue channel shows the highest contrast between object (tree) and background (sky). Using the blue channel as the base for this image is therefore most favorable.
Oak Tree - Image ID: 643191 © Marilyn Barbone 123RF.com
Step 10
From the blending options window, go to “Blend if:” section. Click on the drop down box and select Blue. We are going to knock-out the brightest part of the "blue" channel.
Step 11
Hold ALT to split and drag the right sliders to 70/200 ratio. I’ve used a bright red background this time to aid in the visibility. The values between the sliders will result in an almost transparent background and a smooth blend.
Step 12
You will find traces of cyan/blue (“leftovers” from the sky) on the edge of the object. Its highly unlikely to be able to mask the object perfectly in one go, so improvisation is a must. We can desaturate the cyan in the object a little at a time.
Step 13
Press CTRL+U to bring out the Hue/Saturation window. From the drop down box, select Cyan and desaturate it by 75 unit (-75). 
Step 14
Create an empty layer and merge with the working layer so that it is flattened. Alternatively, you can leave it unflatten so that you can always access the blending option to refine the settings again. Now the image looks better as the “residues” from previous background are blending better.
Step 15
There are plenty of ways to tweak around with the image other than just adjusting the Hue/Saturation. Try Selective Colour and/or Colour Balance. One can even do further masking to ensure a more surreal blending. Example as below.
Skyline - Image ID: 1011284 © Gary Blakeley 123RF.com
Step 16
Another option - the “Underlying Layer” works in a similar way. The difference is the layer beneath (building) the working layer (model) will be revealed when the slider is dragged. Take this two image for example. Place the female portrait layer on top of the building layer.
Building - Image ID: 699373 © Rui Vale De Sousa 123RF.com
Step 17
Transform (CTRL+T) the female portrait to suit the building’s perspective. While in ‘transform’ mode, simply hold down CTRL key and drag all four corners to distort the image.
Step 18
What we want to achieve now is to reveal the frame of window panes and its shadows (as shown in the circle) within the top layer (model). Drag the left sliders to reveal the dark areas of the underlying layer, and drag the right sliders to reveal the bright areas of the underlying layer. Use the Blue Channel option again to gain the best result.
Step 19
Mask away additional clouds covering the model's face. When you are satisfied with the results, merge them. Please note that merging the working layer with an empty layer will nullify “Underlying Layer” sliders (unlike "This Layer" sliders), and thus you would have to tweak it again. Best is to merge them all together (model + building + mask-out face layer).
Blending and masking using this method involves trials and errors. Have fun trying! That’s all for now.
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