? Explain Render Options Alpha and Pre Multiplied Alpha ?

Comments

• Richard Haseltine Posts: 110,630

  June 22

  With pre-mulitplied the semi-transparent pixels get the background colour mixed in - that can give a smoother blend if whatever you ar elayering it over roughly matches the background colour but a glaring halo if not.
• Diomede Posts: 15,436

  June 22

  Thank you
• WendyLuvsCatz Posts: 41,070

  June 22

  something you don't want to do if rendering png series for billboards 

  it's OK if you are adding the same background in post

  if using a big 16K HDRi for example, You can render with a low resolution lighting version 

  then render just the high resolution one as the background 
• Stezza Posts: 9,099

  June 23

  going full Carrara here:

  Every pixel in a rendered image has:

  • R, G, B (color)

  • A (alpha, transparency)

  1. Straight Alpha (un‑premultiplied)

  RGB is stored as if the pixel were opaque, even if it’s actually transparent.

  Example: A pixel that is 50% transparent red:

  • R = 1.0

  • G = 0

  • B = 0

  • A = 0.5

  When compositing, the formula is:

  Cout=Csrc⋅A+Cdst⋅(1−A)

  This works perfectly if the compositor handles straight alpha correctly.

  2. Premultiplied Alpha

  RGB is already multiplied by alpha before saving:

  R′=R⋅A

  So the same pixel becomes:

  • R = 0.5

  • G = 0

  • B = 0

  • A = 0.5

  Now the compositor uses:

  Cout=Csrc+Cdst⋅(1−A)

  This avoids a major problem…

  With straight alpha, the RGB channels still contain the original background color.

  If Carrara rendered your object over black, the transparent edges contain black RGB values.

  When composited over a different background (e.g., sky blue), the black edge leaks through → dark halo.

  With premultiplied alpha, the RGB values are already faded toward transparency.

  There is no leftover background color to leak.

  This is why premultiplied alpha is the standard in:

  • film pipelines

  • OpenEXR

  • linear compositing

  • VFX studios

  Premultiplied alpha is bad for PNG billboards because the RGB gets darkened and you lose edge detail .

  But it’s good when:

  • you’re compositing over the same HDRI you lit with

  • you want film‑style smooth edges

  • you’re rendering EXR sequences

  • you want mathematically correct blending

  In summary:

  Premultiplied alpha prevents color contamination at semi‑transparent edges by ensuring the RGB channels represent actual visible light instead of “imaginary opaque color,” which eliminates halos during compositing.

  yes, I'm bored 
• WendyLuvsCatz Posts: 41,070

  June 23

  oh, I had it the wrong way around 
• Diomede Posts: 15,436

  June 23

  Wow, thank you Dave for the detailed response. Great information.