DIGITAL SESSIONS WELSH SCHOOL OF ARCHITECTURE - CARDIFF UNIVERSITY
PHOTOSHOP: ADVANCED MASKING TECHNIQUES
WASSIM JABI AND SERGIO PINEDA
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DIGITAL SESSIONS WELSH SCHOOL OF ARCHITECTURE – CARDIFF UNIVERSITY DR. WASSIM JABI AND MR. SERGIO PINEDA
PHOTOSHOP: ADVANCED MASKING TECHNIQUES INTRODUCTION The goal of this tutorial is to isolate the tree shown below from its background so that it can be copied and pasted on top of another image. Adobe Photoshop allows you to create and save a mask that defines an area of selection. This tutorial will demonstrate how to extract a clean mask from a photo using channels, and then export the final result as a .PNG file with a transparent background. An image with a transparent background is easier to composite with other images. Channels in Photoshop are similar to layers that combine together to describe the full color and transparency attributes of an image. There are red, blue, green, and transparency (alpha) channels. Each channel describes the amount of that color or attribute in the image. It is represented visually by using a greyscale from white (full color intensity) to black (no color).
STEP 1: BALANCING THE CHANNELS The main challenge here is that while the upper part of the tree contrasts very well against the sky, it is very intricate and difficult to select manually. On the other hand, the trunk in the lower part of the image doesn’t contrast very well against the landscape, but is much easier to select manually using the Marquee selection tool. • Make sure you have the file ‘Tree.jpg’ before starting the tutorial. • Open ‘Tree.jpg’ in Photoshop. • Select Window Channels to open the channels palette and inspect the various colour channels.
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STEP 2: USING COLOUR CHANNELS Colour channels tell you how much of each color is in the image. An RGB image like this photo will contain 3 seperate colour channels, one indicating the amount of Red, one for Green and one for Blue. Look for the channel with the highest contrast edge outlining the tree. In other words, we want to find a sky as white as possible and a tree as black as possible. To see each channel just click on it. For example, this is the green channel:
The green channel still has a lot of grey around the tree. Let’s have a look at the red channel:
The sky is still very grey in the red channel. Let’s see the blue channel:
The blue channel offers the clearest distinction between the two tones, which can be enhanced with other Photoshop tools. 3
STEP 3: ADJUSTING BRIGHTNESS AND CONTRAST In order to use the blue channel as a mask, we need to try to achieve a clean black and white/high contrast image. In order to achieve that, we will use the brightness and contrast tools: • Make a copy of the Blue channel (right click on it and then select ‘Duplicate Channel’). • Select the ‘Blue copy’ channel • Select Image Adjustments Brightness/Contrast… Modify the brightness and contrast of the ‘Blue copy’ channel until the tree is as black as possible and the sky as white as possible.
• Make sure that at the time of moving the sliders for Brightness and contrast the ‘Use Legacy’ box is ticked! • A potentially good solution to what we are looking for could be: Brightness +2, Contrast +65. It may take a number of tries to get the right balance (and obviously, for different images you will probably need different variations of these numbers). • Be mindful of what is happening throughout the image as the brightness/contrast is adjusted. If the white levels get too low, thin parts of the branches will disappear into the sky. And if the black levels get too high, the sky will glow through the edges of the tree. • Drag the ‘Blue copy’ channel onto the ‘Load channel as selection’ button.
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STEP 4: ERASING THE SKY AND LANDSCAPE Go to the Layers palette, and make a copy of the Background layer (right click then select Duplicate Layer…).
• Double-click on the new layer ‘Background copy’, and rename it ‘Tree’. • Hide the ‘Background’ layer. (Click on the eye icon to hide the layer) • Select the ‘Tree’ layer and delete the contents of the selection by pressing the delete button on your keyboard. In effect, you will be deleting the sky from the image.
• Select the Polygonal Lasso Tool and select the trunk of the tree in order to erase the landscape behind it.
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• Now erase the landscape by pressing the Delete button on your keyboard.
What we are going to try to do next is to insert the image into other images (perspective drawings, elevations, etc). 6
STEP 5: USING THE TREE AS A PHOTOSHOP LAYER The easiest option of moving the tree into other drawings is to copy and paste it into another Photoshop file. • Open ‘Elevation.jpg’. This image is the kind that can be produced with programs such as 3D Studio Max or Microstation.
• Now, in the tree image (and with the tree layer selected) Select All (Ctrl-A), Copy (Ctrl-C). Go to the elevation image and Paste (Ctrl-V). • It has obviously come in too large. To adjust, select any of the selection tools.
• Right click on the tree (whilst having it selected as a layer) and select ‘Free Transform’. • Adjust the size of the tree by moving its corners. To maintain its proportion, press the SHIFT key while dragging its corners. You can also click on it and drag it around the image to locate it at your prefered location. • Change the brightness and contrast of the image to suit the balance of the image. And you are done!
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INSERTING THE IMAGE INTO OTHER PROGRAMS To export the image as a bitmap with a transparent background, the .PNG file format provides a good option. PNG files can be imported into other programs such as Illustrator or 3DS Max and they maintain their transparency information. • Go back to the Tree file. • Hide the Background layer. • Select the Tree layer. • Select Select All (Ctrl-A), then Edit Copy (Ctrl-C). • Select File New (Ctrl-N). Make sure the Background Contents is listed as ‘Transparent’. The image size in pixels will be exactly the size of the tree image that has been copied. Do not change those values. • Once the file opens, select Edit Paste (Ctrl-V)
• Select File Save As… Select the PNG file format from the file format list and save the file as tree.png Now you can import the PNG file into Illustrator with a transparent rather than an opaque background.
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Wassim Jabi, Ph.D. is a Senior Lecturer at the Welsh School of Architecture Sergio Pineda, AADip is an Architect and a Professional Tutor at the Welsh School of Architecture