I am always reluctant to provide Photoshop recipes online for a variety of reasons. First and foremost is the recognition that one way of creating an effect can more than likely be accomplished by several other methods. With this caveat known, let’s proceed.
In previous Photo Life articles, I have written about knockout techniques using green-screen technology. I also mentioned that it is possible to shoot on a white screen to achieve similar results. However, I personally prefer the green-screen technique, primarily because the Digital Anarchy plug-in “Primatte” does a very good job of creating the knockout mask.
But I have received several enquiries for my recipe to create alpha masks using a white screen. So I have placed my recipe below with the full understanding there may very well be better, more efficient methods. Feel free to share your own recipes in the comments, should you feel so inclined.
I created this scene in my studio, but I did not use any lighting equipment other than the overhead fluorescent tubes that might be found in any car garage. They provide generally even illumination, and you can then use reflectors and modifiers fashioned from whatever works. Create a custom white balance in your camera, or use an auto white balance and caress it later in your editing software, and voilà…have fun.
White-Screen Knockout Recipe
1. Create a Duplicate Layer, and rename it to whatever your workflow preference provides.
2. Check Channels Red, Green and Blue for the channel with the greatest contrast, and leave that Channel active.
3. Go to Image > Calculations > and ensure blending mode is Multiply and Opacity is 100%. Hit OK. This creates an Alpha Channel named Alpha 1.
4. With Alpha 1 as the active Channel, once again go to Image > Calculations, only this time ensure the Blending Mode is established as Overlay. Notice you now have a second Alpha Channel named Alpha 2.
With most images the content will now be black and white without shades of grey, which is the objective. However, if you think you still have some grey in the white screen, you can increase the contrast more by using levels.
5. With Alpha 2 as the active Channel, hit the Control key and L and the Levels dialogue will appear. Move the Highlight slider to the left until the background screen (seamless paper) becomes white.
It is worth noting that should specular lighting (white highlights) still appear in the image you will be keeping—the non-white background screen—you can paint over these with a black paintbrush. The objective is to ensure everything you wish to retain in the scene is solid black.
6. Next, inverse the Alpha 2 channel by ensuring it is the active channel and use the shortcut Control I.
7. Still ensuring Alpha 2 is the active channel, hold the Control key and simply click on Alpha 2. You should now see the marching ants moving around the selection, but now you have to inverse the selection. The objective is to ensure the white screen is now black and the foreground is white. Anything that is white will be retained.
8. Now you need to exit the Channels tabs and get back to the Layers. You do this by simply clicking on the renamed duplicate layer you created.
9. Finally, go to Layer > Layer Mask > Reveal Selection.
10. Create a new canvas with a transparent background, at the desired resolution, and then drag the previously revealed selection into this new canvas. Save as a PSD file to retain the transparent background.
11. Congratulations, you’ve finished the alpha-mask technique of removing scenes from white backgrounds.
As you use this technique more often, you will discover tricks that will help you get clean edges without light spill. For example, at Step 7, once the selection has been made, it is often desirable to go to Select > Modify > Contract and insert 3 pixels as a starting point. Next go to Select > Modify > Feather and insert 2 pixels. By doing this before inverting the selection, it will “tighten” the edges that might have highlights or spill light that will show when the scene is moved into the new background to complete the composite. I have not used Photoshop 6, but I understand the Refine Edges function does a much better job than previous versions. It is worth exploring if you are running the latest version of PS.
It is best to experiment with this technique using a simple foreground element that has hard edges that are non-reflective. Keep it simple as you are starting out.
This technique also works with landscape images that were made on overcast days and yielded a grey, white or otherwise washed-out sky. Many photographers have developed libraries of “elements,” in particular, skies of all imaginable types. They can, and do, salvage some great shots that previously included ugly grey skies.
Have fun, and as with any cooking recipe, season to your personal taste. The only right way is your way.
Next Wednesday I’ll share how I achieved the final result and explain the reasons I treated the image the way I did. Tune in, same channel, same time.
For Part II in this series, click here.