What is the best way to render DOF blur over a gradient background?

[Josh Richman]

What is the best way to render DOF blur over a gradient background?

The transparency of the blurring DOF picks up the rendering environment background and the edge is visable. My background will be a gradient. What is the best technique for solving this?

Is it best to composit the image with DOF with a background? How?

OR

Rrender the image with background all in program.?

[DriesV]

I am not sure I understand the context.
Can you provide some images of what you are trying to achieve?

Dries

[mattjgerard]

I think he is asking what is the best way to render out DOF over a gradient background-

1) Render with no background and with an alpha transparency and composite in Photoshop or
2) Render with the gradient background in keyshot to a flat image

I would hazard a guess that your best bet would be to render out with no background, depending on your gradient color premultiply that alpha with black or white, and composite in photoshop.

I'm always a fan of allowing more control in post production, as its a lot faster to tweak the look in PS than to re-render the whole image to change one small thing. This is me coming from a motion graphics video background though, and doing all my compositing in After Effects was a whole lot more flexible than trying to get a final product straight out of my 3D program (C4D) in one go. Made changes and tweaking a whole lot faster and more flexible.

[Josh Richman]

Great advice!

But one big question. How do I render out without a background?

Like I mentioned above, the blurred depth of field sections that are transparent pick up the background. So is best practice to render on white / black / 50% grey ?   Because the final image is composited on a gradient blue background.

[INNEO_MWo]

I would use a 50% grey color background in KeyShot and render a png with alpha.

[mattjgerard]

I would use a 50% grey color background in KeyShot and render a png with alpha.

I would agree with this, although I have had to expierment a bit with it depending on what my background is. I've played with straight alphas and have to  use the Defringe tool in photoshop, that gives me a clean edge if I'm compositing over other stock imagry. 95% of my stuff is products over white, so for me its simple, I use white as a background color. I render with transparency as well, but I render to PSD files instead of PNG's, just depends on your preferences. I have yet to play with the 32bit PSD's not sure if this will help in post when it comes to DOF and alphas.