Combining Images

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

Combining Images

Post by Robert Schleif »

What is a good way to add or subtract, say, 85% (brightness) of image A to 15% of image B, and then vary these amounts a bit to observe the results? Composite or Blend would be great for this if each contained one more control, Input Amount. Such an addition would also make the use of the sliders in Composite more intuitive. (Currently, it is easy to expect that the Amount slider in Composite refers to the amount of the input image that will be used.)
tomczak
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Re: Combining Images

Post by tomczak »

Would Stack work for that?
Maciej Tomczak
Phototramp.com
jsachs
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Re: Combining Images

Post by jsachs »

Blend gives you a weighted average of the two images. The slider varies from 100% image A, 0% image B to 0% image A, 100% image B. If the weights don't add to 100%, you just darken or lighten the result.

Blend in Absolute or Offset Difference mode subtracts images.

Also, the Difference transformation does subtraction in various ways.

Inverting image B and adding it is like subtraction with a 50% offset.

As Maciej points out, for adding you can use Stack Images which gives you an amount slider for each image.
Jonathan Sachs
Digital Light & Color
Robert Schleif
Posts: 340
Joined: May 1st, 2009, 8:28 pm

Re: Combining Images

Post by Robert Schleif »

Great. Blend using the Blend option is good for the addition operation, and using the Subtract option is good for the addition operation.
Use of the Blend transformation would be much easier if the mathematical operation of the chosen option were displayed. For example, if the Blend option is chosen, and the slider is set at the 25% position, instead of showing "Amount 25%" above the slider, we saw, 75% Input + 25% Overlay, and for the Add option, we saw 100% Input + 25% Overlay and so on. By the way, not even the documentation gives me any idea what the option Reverse Soft Light does since I've never used any image editor except PWP.
jsachs
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Re: Combining Images

Post by jsachs »

In all the modes, and in all other transformations, the Amount slider works the same way -- it produces a weighted average of the transformation result and the input image. At 100%, you get the full force of the transformation, and at 0% you get the input image. At 50% you get half of each. It also works the same way with masks.

It would not be good design to have the Amount slider work differently in one transformation.
Jonathan Sachs
Digital Light & Color
Robert Schleif
Posts: 340
Joined: May 1st, 2009, 8:28 pm

Re: Combining Images

Post by Robert Schleif »

At last, I'm beginning to understand. When I only partially understood, the Blend option confused me because for it, the amount slider changes the contributions of both the input and overlay images. It seems that for the other options of the Blend transformation (those that I can understand) the amount slider changes only the contribution from the overlay image. Some way of indicating this difference may be helpful to others.
jsachs
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Re: Combining Images

Post by jsachs »

The Amount slider does the same thing in all modes, but perhaps Blend should be called Copy since at 100% you get a copy of image b. Then the Amount gives you a percentage of the copy combined with 100-percent of image A.
Jonathan Sachs
Digital Light & Color
tomczak
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Re: Combining Images

Post by tomczak »

I need to confess that the Composite gets me every time, even though the way it works makes perfect sense. I've been trying to make up a physical analogy that I can remember and relate to so that I can predict what the transformation will do, especially with masks. This is the best I can think of - perhaps there are some better ones.

Imagine three slide projectors A, B and C, all casting their images onto the same screen.

Projectors A and B are loaded with the same Input image.
Projector C is loaded with the Overlay image.

Overlay Amount Slider: The total amount of light projected by B and C together is constant. The Overlay Amount slider balances the relative contributions of B vs. C: at 0% all the light comes from B (so it makes no difference what the Amount slider is doing as A and B are the same), at 100% all the light comes from C.

Amount Slider: The total amount of light projected by A and B+C is also constant. The Amount slider balances the relative contributions of A vs. B+C: at 0% all the light comes from A (so it makes no difference what the Overlay Amount slider is doing), at 100% all the light comes form B+C.
Maciej Tomczak
Phototramp.com
jsachs
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Joined: January 22nd, 2009, 11:03 pm

Re: Combining Images

Post by jsachs »

I don't know if this will help or not...

Simple Rule:

The Amount controls what parts of the base image are covered by the overlay image, and by what percentage. 0% gives you the base image, 100% gives you the overlay image, unless it is smaller than the base image, in which case part of the base image shows through.

The Overlay Amount controls the opacity of the overlay image. The Overlay Amount mask, if any, is shifted, rotated, scaled or warped along with the overlay image, depending on the number and placement of any alignment points.

One Image:

The overlay image is a solid color, and there is no Amount mask. The two amounts are multiplied - 50% amount and 50% overlay amount applies 50% of 50% or 25% of the overlay color. If there is an Amount mask, the solid color is only applied where the mask is not black.

Two Images:

Without masks, again the two amounts are just multiplied. The only difference is that the Amount applies to the entire base image while the Overlay Amount applies only to the overlay image. With masks, the amount and overlay amount vary with the masks.
Jonathan Sachs
Digital Light & Color
tomczak
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Re: Combining Images

Post by tomczak »

Think of Blend transformation (alignments and Overlay masks aside): this is Composite with projector B switched off.
Maciej Tomczak
Phototramp.com
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