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Another question on alpha channels and transparency in Photoshop.

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I have a partly transparent 3d-object which I have rendered in TrueSpace against a black background. If I save the rendered scene in 32 bit TGA-format, an alpha channel with transparency information is included. Nice... However, if this channel is applied to the rendered picture the result will, of course, not look right as the transparency of this picture already has been calculated and then been applied on a black background. What I really need is a sort of original picture with no black background to which I can apply the alpha channel. The resulting picture overlaying a black background should then be identical to the rendered scene. Is there a way to do this? I have experimented a little, trying to remove the blackness with the help of the alpha channel but I have not been able to get it 100% right. Sorry if this makes no sense Perhaps I should try a totally different approach?

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Hmm... so my guess is you are getting a "black line" around your alpha''d images? and thats wha tyou want to get rid of?

This sounds to me like a problem with your lighting. Make sure you have plenty of backlight

You are still going to get a little bit of black, but that''ll be because of antialiasing.

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Ok, obviously I didn''t make sense

This is the original rendered image (two glass pipes against a black background):







This is the alpha channel with transparency information:







This is the result of applying the alpha channel on the original image (alpha channel is loaded as selection and this selection is cut out from the original image).







This is the same image but now on black background.







As you can see it is darker than the original rendered image. Naturally since the black background from the original image was included when the alpha channel selction was cut out. So the original image and its alpha channel will not look right when overlaying backgrounds of different colors - they will always be too black. If I render against white background, they will on the other hand always be too white.




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Have you tried opening your rendered scene in another photoshop window alongside your image of the glass tubes on a transparent background and then using the clone tool from that window to the other window with your render in? I''m pretty sure that would work.

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wow, actually an original question about transparency

I have a feeling that you just need to play with the blending style.
If you do a normal blending, by only applying the alpha channel to your pic, you would get what you are showing.
But what about using the Screen mode ?
Or you could compensate for the black background in the original by using the Adjust > Hue/Saturation menu and make your colour layers brighter (without changing the alpha layer) ?

See, the operation to apply depends on how the transparency in the original picture works. So I am not sure there is a clear cut answer to your question there, only tweaking and fiddling with the controls until you get it right


Sancte Isidore ora pro nobis !

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Hah hah, I know what your problem is. I explained it in either this post, or your last one... but I''ll try to re-explain it.

Okay, the pipes are transparent, right? Well, they are up against a black background. All your transparent colors are being mixed with black.

So, you have a pixel that is of 50% transparency blue. It get mixed with the background, so its now 50% blue, 50% black. That same pixel is defined as 50% transparent in the alpha channel. When you key it, it will be 50% of the new background, 25% blue, 25% black. This is what is washing out your image.

The fix? Render out an opaque version of your pipes, and use the transparent one''s alpha channel.

With keyable graphics, the actual image has to be completely opaque, define all transparency in the alpha channel.

- T

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I know there was an easy solution... It works fine (for some reason the colors of the opaque pipe is very unsaturated compared to the transparant one, but that is easy to adjust). Thanks!

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