Help with alpha composite

[Kevin Lockwood]

I'm working on a project where I am rending a shade structure that I want to composite onto a site photo. I've set a ground plane and tagged it as alpha channel, but what I get isn't how I think it should look. I'm not getting any transparency in the shadow (well, just a tad maybe).

I've done this in the past and I think all my settings are about the same. What am I doing wrong??

Samples: (Click to enlarge)

Image while rending. I think the items tagged as alpha used to show here, but I cant remember for sure.

Render window shows some transparency, but not what I would expect

When rendered without tagging the ground plane as alpha, shadows look pretty normal.

Compositing in photoshop... ick!

Composite using render on ground plane not tagged and using sky alpha, masking using materials channel, and masking just around the shadow. Desired look with a lot more work to get it.

[Peter Milner]

I think it's important that you assign a material to the ground plane similar to the photo you're compositing with.

I did a quick crude test with no materials assigned and you can see that the shadow is definitely the wrong colour. However I do get the transparency, though probably too much.

[Peter Milner]

This one is purely rendered with a background image (no Photoshop involved).

[Peter Milner]

This is rendered with background image and then composited in Photoshop.

Rendering with the background image gives you the opportunity to adjust the lighting and perspective of you render to match the surroundings.

[George Ioannidis]

Sahara table. Brand-new stuff from Sven Christiansen Peter? :D

 

Nice paradigm with explanation. Thanks.

[Roy Hirshkowitz]

Edit the mask channel in PhotoShop so you get more transparency-- simple brightness adjustments will probably work fine.  When compositing make sure that the color your compositing with is darker (black, for a start).

[Kevin Lockwood]

I did try using a material similar to the photo I was matching, but not with the background, as Peter suggested. I'll give that a go. I also used a solid background, but should have gone darker as you suggested, not lighter.

 

This is one I did when alpha compositing was first introduced. Worked great for me back then :)

[Roy Hirshkowitz]

That one worked as is because you're compositing with white.

 

The visible background you use while rendering is not relevant-- other than it may effect the exposure of the image-- which can be reset using the Brightness setting.  (Reflective backgrounds may be important-- depends on the image.)  (Peter is probably right in that it's easier to adjust things if you see everything at the same time.)

 

******

 

When compositing you have complete control.  You can save off as many mask channels as you want in the image editor and edit them independently.  You decide what colors to combine using photoshop, not the rendering background.  The renderer encodes how much occlusion there is into the alpha channel when creating those shadows-- it's just a guess though (there is no way to really do this)-- and you may need to adjust the shadow portion of the mask channel.

 

If you want to include the nXtImage of the thing I can take a run at it.

[Roy Hirshkowitz]

There's definitely something wrong with the alpha channel objects in the more recent builds--  I'm looking into it.

[Kevin Lockwood]

Thanks Roy. I tried all the tips given above and was getting the same results. Thought it was just me :)