Advanced Masking Question

[Rnld]

Hey guys,

I'm working on a series of photos at the moment. I have an object and behind it a very busy leopard print background. I photograph the object from a tripod with studio lighting.
Is it possible to make 1 photo with the object in it, and one without (just the background.) and somehow let photoshop calculate a mask with the 2 different photos?

I know about the "difference" blending mode, but I'm not entirely sure how to use it for this purpose. Do you guys know any good ways to do this, or am I just lazy and should I just the pen tool? :P


thanks!

[philbach]

Well on your photo of the object you can place a green screen or a blue screen behind the object like they do on video and that would make it easy to delete the background from the object picture.

[Repairman]

If you are doing a series of small low value shots (ie for a catalogue) you can get away with using an auto mask on the object (magic wand or masking software) If the images are core images for advertising and will be multi purposed for varied media I would make the masks by hand. It's often quicker to make a good mask once by hand than to edit a poor automask. Depends on your budget and expectations though.

[Rnld]

Thanks for the replies.
I know how to mask with selectiontools, channels and the pentool. I was just curious if there is a way to use 2 photos and have photoshop select the difference between those 2. I can imagine that would come in handy sometimes.

[John Wheeler]

Hi Rnid
I have used the technique you are referring to yet there can be issues.

First, the shot needs to be aligned exactly down to the pixel (e.g. shots on a tripod). You may need to use the align layers feature as well. The lighting on the background should not change when you pull the object out. Also, the focus point needs to be the same for both shots (e.g manual focus).

Without the above, the difference image will not be very exact and a lot of extra work to compensate.

If those conditions are all in place here are some steps that could work

- Shot with object
- Shot without object and only background on next layer up set to difference mode
- Use threshold adjustment layer and adjust until background is all black yet the object is white (note, if parts of the object match in RGB values with the background those points will also show as black as well and will have to be painted white later
- Optional - used the blend if sliders to help tune in the black and white image
- Cut and paste the B&W image to a mask
- Hand paint corrections as needed

So those are the steps to take yet may be a lot more work than the approaches mentioned by Philback and Repairman. Hope that helps

[Rnld]

Thank you John, I might try it because I have made the photo's this way.
However I do think it is easier to just make a nice path with the pen tool

Thanks for the feedback!

[Mike Abbott]

Rnld,
Just one other approach to this, which may or may not be be practical or worthwhile, depending upon the exact setup you have...

Take your primary image.
Take a secondary image as follows: Drop some white seamless down just in front of your background. Light so background is white, object is in silhouette.
Cut and paste your secondary image into a layer mask of the primary image. Invert mask.

Mike A.

[Markzebra]

Avoiding all the shooting advice that you have received - Yes you can easily calculate the differences between two layers in Photoshop. There are various ways to do this. Inverting one layer and taking it down to 50% opacity, Difference Mode, or Apply Image can be used. Then Color range (old style not "localized color clusters") can be used to establish the 50% gray. A Mask can be generated from this.