Understanding transparency

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Old 10 October 2012   #1
Understanding transparency

Hi people. I am looking into the transparency of an object in the alpha channel.

It seems C4D has a non-linear relationship with transparency of an object and the final luma value in the alpha channel. This is causing me some problems as I am having to render a glass logo separate to the background for comping later. So the logo is nice and glass like, but the alpha is basically solid. Not good for comping and seeing stuff flying behind.

I don't have this with 3DSmax, which does deal with materials quite differently.

So have any of you come up with a nice method to have an alpha channel that more correctly reflects the transparency of an object?

I have used the transparency render pass, which has some validity, but it does have problems where there is a solid object mixed up with the transparent object.
Gareth Qually
Slowly Making Smoke

Old 10 October 2012   #2
Use the Transparency channel instead of alpha. If objects are showing through the transparent object in C4D, then they'll show up in post too (minus the refractions).
Old 10 October 2012   #3
I'm not sure what the problem is but here, using object buffers, alphas are solid and transparent at the right places, even stacking transparent and alpha materials. do you have an example file?
Old 10 October 2012   #4
Originally Posted by eikonoklastes: Use the Transparency channel instead of alpha. If objects are showing through the transparent object in C4D, then they'll show up in post too (minus the refractions).

Oh I am using the transparency channel. I think I should put up a pic to better explain what I mean. I will do this tomorrow.
Gareth Qually
Slowly Making Smoke

Old 10 October 2012   #5
Ok here is a graph of my test. So I added a cube with a plain material, only luminance at 100% and animated the transparency brightness in 10% increments (stepped keyframes) over 11 frames. This is a graph of the transparency value and the resultant alpha channel values.

So it seems that we have a non linear graph, with this weird dip at the end. So you can see that at a 60% transparency brightness, we are sitting at roughly 90% alpha.

It would be nice to switch to a linear method if needed.
Attached Images
File Type: png Screen shot 2012-10-03 at 11.25.56 AM.png (23.5 KB, 29 views)
File Type: jpg transparency test_00000.jpg (14.7 KB, 25 views)

Last edited by fxgogo : 10 October 2012 at 09:45 AM.
Old 10 October 2012   #6
hi fxgogo,
when reading the transparency value of a cube, your adding the value of two sides of that cube against whatever background you have. I did your test with a plane, animating a color texture in the alpha channel from white to black in ten steps, and reading the black values in photoshop, the results aren't that dramatic but I agree that isn't linear, I'll have to check if it isn't due to the key frame interpolation that isn't linear by default but "spline".


Last edited by laurent : 10 October 2012 at 05:05 PM.
Old 10 October 2012   #7
Not at my computer to try this, but could it be Linear Work Flow affecting this? I seem to remember putting a gradient on a plane going from black to white and noticing that the mid point (50% grey) had swayed off centre- side effect of the gamma effect?. May relate to a transparency gradients and explain your issue.
Old 10 October 2012   #8
I think the issue is that c4d apply the linear treatment on the transparency channel but not on the alpha channel. To "fix" the issue either turn of linear workflow in the project settings or change the input color profile to linear or disabled from the default setting of sRGB.

But its something weird going on:

Old 10 October 2012   #9
I double checked to see if I was using linear workflow, but no I am not.

I have had that issue in the past with using bitmaps, and having to specify the colour profile or else you do get strange results, especially when bitmaps are driving animation in mograph.

I did do some more tests, and it seems the saturation in the transparency channel has a huge effect on the alpha value of the glass. I expected the the value slider to be the controller of opacity and saturation would just control...well....saturation, and to a lesser extent opacity.

Dropping the saturation from 80% to 50% had a huge affect on transparency. Check the attached picture. The good thing is the green bars in the back ground match up (the lower one is comped in after effects).
Attached Images
File Type: png Screen shot 2012-10-04 at 11.32.41 AM.png (14.8 KB, 13 views)
Gareth Qually
Slowly Making Smoke

Old 10 October 2012   #10
Thats interesting Gareth.
Saturation is a funny thing, it can also cause tonal 'blowouts' a bit like specular blowouts but on the mid tones. I guess that says something about it's nature and how it can affect a range of tints. It seems that high saturation can sway things off a realistic result. Its the same in Photoshop.
But no such issues in Max?
Old 10 October 2012   #11
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