Photoshop and alpha

[Guest]

I have rendered out an object on a vray plane along with an alpha channel which has shadow information in it. the object is rendered on a black environment background.

How do i use the alpha channel to cut out the object and shadow information so i can over lay it onto a different background image?

I used to render on as PNG which when opened in Photoshop automatically got rid of any black environment, but I’m using tiffs and all my elements have black backgrounds.

Thanks,

[Guest]

Do i have to collapse my stack of render elements before i can use a clipping mask?

[joconnell]

Go to your channels in photoshop, Ctrl + click on the alpha channel and it'll load the transparency as a selection. Click on the rgb channel, go back to your layers and make sure you have your render highlighted. Then hit Ctrl + J (new layer via copy) to copy the selected pixels to a new layer using the alpha selection as transparency. Alternatively, duplicate your background layer so your render sits on layer 1, add a mask to the layer, go to your alpha in your channels. Select all, copy this then go back to your layers. Alt click (I think) on the mask you added so you're viewing the mask (should be all white) and then paste in your alpha channel. If you turn off your background layer, you should now have transparency for the shadow parts and solid pixels for the non shadow parts of your render.

Only thing I'd say is that it's not as easy to control how thick your shadow is if it's in the same layer as your main render. If you play with the grey values of the alpha channel to tweak the shadow, it'll also affect the semi transparent pixels along the anti aliased edge of your object. Handier to have your main render with no shadow, then a shadow render with the object hidden to camera.

[Guest]

"Then hit Ctrl + J (new layer via copy)" So I have to collapse the stack first and loose the ability to edit elements? I cant just use the alpha channel mask to mask all my elements in one hit?

[joconnell]

Sure - put the elements into a layer folder and then put your mask on to that.

[Guest]

I still dont follow, i have attached a screen shot of my stack and i cannot work out how to mask each layer in one go without collapsing the stack first. i have groups within groups, does that affect the result?

[Dariusz Makowski (Dadal)]

Turn on that 1st layer with alpha

M > CTRL A> CTRL C
Go to channel
New layer > CTRL V for paste
CTRL L click on thunbail to select pixels
Go to back to your layers, and use it as mask.

Done.