On 5/28/19 1:02 PM, Julien Hardelin wrote:
Hi,
I am trying to update Color to alpha, but I don't understand
transparency and opacity thresholds.
Pop-up helps mention a limit that I suppose to be a transparency limit.
"below" and "above" seem to me inverted.
I don't see how to describe these options.
Please give me explanations with, as much as possible, examples.
Color to Alpha modifies the transparency (and color) of the pixels based
on their distance from the selected background color (the "Color"
option") -- the closer they are to the background color, the more
transparent they become, with the background color becoming fully
transparent. It's normally done in such a way that recomposing the
result against the background color reproduces the original image.
The transparency and opacity thresholds control how close colors should
be to the background color before they become fully transparent, and how
far they should be from the background color before they remain fully
opaque, respectively. With the default values of 0 and 1, only the
background color becomes fully transparent, and only the colors farthest
away from the background color remain fully opaque.
For example, while the default values work well for removing a white
background from a black object, if the object is gray instead [1] it
will become semi-transparent [2], since gray is midway between white and
black. Lowering the opacity threshold to 0.5 fixes that, by keeping all
pixels that are gray or darker (all pixels whose distance from white is
0.5 or more, on a [0,1] scale) fully opaque [3].
The transparency threshold works similarly: raising it causes more
colors in the neighborhood of the background color to become fully
transparent. This is mostly useful with noisy images, in which the
background is not fully solid. However, unlike in other cases, when the
transparency threshold is above 0, recomposing the result against the
background color no longer reproduces the exact same image.
At the risk of being a bit technical, this can be visualized by thinking
of the RGB cube. The background color is a point within the cube, and
the transparency and opacity thresholds are two sub-cubes centered
around the background color. Everything inside the
transparency-threshold cube becomes fully transparent, everything
outside the opacity-threshold cube remains fully opaque, and everything
in between gradually transitions from transparent to opaque. In image
[4] you can see the Red-Green face of the RGB cube. (1) is the
background color (Red=0.5, Green=0.5, Blue=0.0), (2) is the transparency
threshold (set to 0.1), and (3) is the opacity threshold (set to 0.4).
Hopefully this helps :)
[1] https://i.imgur.com/4dr2kvx.png
[2] https://i.imgur.com/2lbMKIK.png
[3] https://i.imgur.com/fx5EeB3.png
[4] https://i.imgur.com/QocaTFH.png
--
Ell
