Alpha Mask

Kerkythea 2007

How to use Mask Render

Revision Date: 4 February 2007
by Shane L. Fletcher - “Fletch”

A “Mask Render,” also known as an “Alpha Mask,” is a black and white or greyscale image
used to hide or obscure portions of an image without actually losing any information. In
Kerkythea an alpha mask is loaded into the ‘Clipping’ portion of a material to make part of a
material imagetransparent or semi-transparent.
Other common uses:
- place a background image
- place an object partially hidden ‘inside’ your 2D image

PART 1: CREATING THE MASK
To create a mask render of an object, select the object in Kerkythea, and render it using
Mask Render.
1. Select an object or objects
- you can select an object by clicking on it in the view
- you can select an object byclicking on it in the object list on the left pane
- add to your selection by holding the CTRL key
- CTRL-clicking an object a second time will drop it from the selection set
- Select All objects in your scene quickly by clicking the top-most item in the object
pane on the left, hold down SHIFT key and click on the bottom-most object.
2. Click the ‘Start Render’ button

3. Resolution: must bethe exact size of the color rendering you are going to mask
4. Settings: ‘25. Mask Render’
5. Open the image window

6. Any object you have selected will render as solid white. Your rendering could look
something like this:

7. Click “Save” and save it… this one was called “Living Room Alpha”

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8. If you haven’t done so, render the full final image at the exact same resolution youused
for your alpha, being sure to turn on Gamma Correction setting it to 1 and 2.2 if it is
not already done for you automatically.
9. This one was completed using the ‘Clay Render GI-Medium’ feature. This feature is used
to test lighting conditions while ignoring your materials with the exception of emitters and
transparent materials. In this case, it will also easily show the inserted colorbackground.

10. If Gamma Correction is turned off, your scene may look dark, like this:

11. Save the image, in this case the image was called ‘Clay Render_Gamma2.2’
“My Mask Render Turned Out Completely White”:
When rendering an alpha mask for interiors backgrounds, you will want to select everything
except your exterior glass… (or you will get a mask that is just a white screen).Important Issue SketchUp Users May Run Into:
“I selected all the objects in my scene. Then I deselected my glass by CTRL-clicking it in the
view, but the mask still rendered white”
Solution: In SketchUp the reverse(blue) side of your glass may have been painted
with a different glass material. In this case, go to your SU file and see if this is true,
and which material it is. Then find anddeselect that material by CTRL-clicking it in
the object selection pane on the left before running the mask render.
Lots of Materials in Your Model and They Are All Mixed Up?:
If all your materials are not in alphabetical order in the object list, click on “Models” in the object
list. When the word is highlighted, right-click and choose “Sort”.

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PART 2: MASKING THE IMAGE
Photo Editor Time!For the purpose of this demonstration Photoshop CS2 is being used, but this is only one of many
programs available to do this. The Gimp is a freeware open-source program that is also capable
of this.
Warning – THIS IS EASY to do, complicated to explain, and takes a little practice.
Masks are possibly the most powerful and under-utilized feature in Photoshop. This will appear
complex to abeginner user of either Kerkythea or Photoshop, but after following these steps 3 or
4 times, masking will become second nature and can easily be accomplished in under 60
seconds – including rendering the mask. (It sure beats trying to cut out the image manually in
Photoshop using the magic wand or magnetic lasso or some third-party masking software!)
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Open the...