Sliders in Composite Transform

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Robert Schleif
Posts: 340
Joined: May 1st, 2009, 8:28 pm

Sliders in Composite Transform

Post by Robert Schleif »

Although there is an Amount slider apparently associated with the input (base) image and an Overlay Amount slider apparently associated with the overlay image, under every condition that I have tried, I find that both sliders do the same thing and adjust only the amount of the overlay image utilized in forming the final composite image. It seems more logical, and would be more useful to me, if the upper slider, the Amount slider, controlled the amount of the input image, and the lower slider, the Overlay Amount slider, controlled the amount of the overlay image.
jsachs
Posts: 4203
Joined: January 22nd, 2009, 11:03 pm

Re: Sliders in Composite Transform

Post by jsachs »

It does work that way already.

The difference between the two amount control mostly comes into play when you use alignment points to place the overlay image over the base image. The base amount sets what part of the base image will be affected by the overlay. The overlay amount sets what part of the overlay image is applied to the base image. For example, if the overlay image contains an image of the moon, the overlay mask would isolate the moon. If the base image contained an image of a tree, you could create a mask to place the moon overlay behind the tree.
Jonathan Sachs
Digital Light & Color
Robert Schleif
Posts: 340
Joined: May 1st, 2009, 8:28 pm

Re: Sliders in Composite Transform

Post by Robert Schleif »

Possibly more descriptive titles of the sliders would help, but the overall behavior when the sliders are set below 100% is pretty complicated and was not obvious to me with simple diddling of the sliders. One of the complexities is that if either or both sliders are set to values other than 0 or 100, an infinite number of combinations of slider amounts will give the same result. This can be seen in what I believe is the mathematical description of what the sliders do. In the blend mode, and in the region of the input image overlapped by the overlay image, the composite consists of

Input image x (1 - Amount slider x Overlay Amount) + Overlay image x Amount slider x Overlay Amount.

To me, a much more intuitive and useful behavior would be to have the entire output image (overlapped area and nonoverlapped area) be described as

Input image x Amount slider + Overlay image x Overlay amount.
jsachs
Posts: 4203
Joined: January 22nd, 2009, 11:03 pm

Re: Sliders in Composite Transform

Post by jsachs »

If you are not using masks, there is no reason to adjust both sliders - just leave one or the other at 100% and adjust the other one. Since amount controls are the only way to apply masks, two separate amount controls are necessary. To make the amounts work with masks, I believe they have to work the way they currently do to be consistent. The math you are proposing can result in values outside the range from 0 .. 255. Currently, if I am not mistaken, I believe the two amounts are multiplied together to get an effective single amount value amt = (base_amt * overlay_amount)/255. Since multiplication is commutative (a * b = b * a), this explains why you get the same result whichever one you adjust.
Jonathan Sachs
Digital Light & Color
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