<div dir="ltr">Thanks for answering Aivar,<div><div class="gmail_extra"><br></div><div class="gmail_extra">I think what your reply did for me is to have me take a step back and consider what we're modeling. If you look at my replies below, I think that the best solution is to use a model where the background is white and each successive layer filters out some of that background, like a gel. A layer attenuates the underlying layer by a fraction of (1 - alpha/255 * (1 - red/255)), resulting in no attenuation for 255 and attenuation of alpha/255 for zero. We can then use a red converter that returns a value of 255 for the blue and green channels and the model and math work correctly.</div>
<div class="gmail_extra"><br><div class="gmail_quote">On Mon, Jul 15, 2013 at 1:59 PM, Aivar Grislis <span dir="ltr"><<a href="mailto:grislis@wisc.edu" target="_blank">grislis@wisc.edu</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<div bgcolor="#FFFFFF" text="#000000">
<div>
<blockquote type="cite">I have an ImgPlus backed by an RGB
PlanarImg of UnsignedByteType and ARGBType.alpha(value) is 255
for all of them, so aSum is 765. It would appear that the
correct solution would be to divide aSum by 3.</blockquote>
Isn't it unusual to define an alpha for each color component,
generally you have a single A associated with a combined RGB? So
averaging the three alphas might make sense here, because I think
they should all be the same value.<br></div></div></blockquote><div>I think you're right, the model always is that each pixel has an alpha value that applies to R, G and B. The image I was using was the Clown example image. DefaultDatasetView.initializeView constructs three RealLUTConverters for the projector, one for red, one for green and one for blue which sends you down this rabbit hole.</div>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div bgcolor="#FFFFFF" text="#000000"><div>
<blockquote type="cite">In addition, there's no scaling of the
individual red, green and blue values by their channel's alpha.
If the input were two index-color images, each of which had
different alphas, the code should multiply the r, g and b values
by the alphas before summing and then divide by the total alpha
in the end. The alpha in this case *should* be the sum of alphas
divided by the number of channels.</blockquote>
I think alpha processing is more cumulative, done layer by layer
in some defined layer order. For a given pixel say the current
output pixel value is ARGB1 and you are compositing a second image
with value ARGB2 on top of it: For the red channel the output
color should be ((255 - alpha(ARGB2)) * red(ARGB1) + alpha(ARGB2)
* red(ARGB2)) / 255. The alpha of ARGB1 is not involved.<br></div></div></blockquote><div>I think that's a valid interpretation. I've always used (alpha(ARGB1) * red(ARGB1) + alpha(ARGB2) * red(ARGB2)) / (alpha(ARGB1) + alpha(ARGB2)) because I assumed the alpha indicated the</div>
<div>strength of the blending of each source. In any case, the code as it stands doesn't do either of these.</div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div bgcolor="#FFFFFF" text="#000000">
<div>
<br>
In other words, if you add a layer that is completely opaque you
no longer have to consider any of the colors or alpha values
underneath it. <br></div></div></blockquote><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div bgcolor="#FFFFFF" text="#000000"><div>
<br>
I think the bigger issue here is this code is specifically
designed to composite red, green and blue image layers. It's a
special case since for a given pixel the red comes from the red
layer, blue from blue layer, and green from green layer. These
layers shouldn't be completely opaque, since the colors wouldn't
combine at all then or completely transparent since then they
wouldn't contribute any color. I don't think transparency is
useful here.<br></div></div></blockquote><div>So this is an argument for blending instead of layering - transparency would be useful if the images were blended and treated as if on a par with each other, allowing the user to emphasize one channel or the other. </div>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div bgcolor="#FFFFFF" text="#000000"><div>
<br>
It's also possible that a multichannel image with > 3 channels
is being displayed with more color channels, namely cyan, magenta,
and yellow. The code here is designed to stop overflow, but I'm
not convinced those extended color channels would combine
meaningfully.<br>
<br>
Aivar<br>
<br>
<blockquote type="cite">In addition, there's no scaling of the
individual red, green and blue values by their channel's alpha.
If the input were two index-color images, each of which had
different alphas, the code should multiply the r, g and b values
by the alphas before summing and then divide by the total alpha
in the end. The alpha in this case *should* be the sum of alphas
divided by the number of channels.</blockquote>
I think alpha processing is cumulative layer by layer. <br>
<br>
This brings up some interesting questions:<br>
<br>
1) If the first, bottom-most layer is transparent, what color
should show through? Black, white? Or perhaps it's best to
ignore this base layer transparency.<br></div></div></blockquote><div>Maybe the model should be that the background is white and successive layers are like gel filters on top. In that case, you'd have:</div><div>
red = (255 - alpha(ARGB2) *(255 - red(ARGB2))/255) * red(ARGB1) </div><div><br></div><div>And maybe that points to what the true solution is. For the default, we could change things so that red channel would have blue = 255 and green = 255 and the first composition would change only the red channel.</div>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div bgcolor="#FFFFFF" text="#000000"><div>
<br>
2) If you wanted to composite several transparent images, how do
you calculate the transparency of the composite? I'm not sure
this is something we need to do.<br>
<br>
Aivar<br>
<br>
<br>
On 7/15/13 10:31 AM, Lee Kamentsky wrote:<br>
</div>
<blockquote type="cite">
<div dir="ltr">
<div>Hi all, </div>
<div>I'm looking at the code for
net.imglib2.display.CompositeXYProjector and as I step through
it, it's clear that the alpha calculation isn't being handled
correctly. Here's the code as it stands now, line 190 roughly:</div>
<div><br>
</div>
<div><span> </span>for ( int i = 0; i < size; i++ )</div>
<div><span> </span>{</div>
<div><span> </span>sourceRandomAccess.setPosition(
currentPositions[ i ], dimIndex );</div>
<div><span> </span>currentConverters[ i ].convert(
sourceRandomAccess.get(), bi );</div>
<div><span> </span>// accumulate converted result</div>
<div><span> </span>final int value = bi.get();</div>
<div><span> </span>final int a = ARGBType.alpha( value
);</div>
<div><span> </span>final int r = ARGBType.red( value
);</div>
<div><span> </span>final int g = ARGBType.green( value
);</div>
<div><span> </span>final int b = ARGBType.blue( value
);</div>
<div><span> </span>aSum += a;</div>
<div><span> </span>rSum += r;</div>
<div><span> </span>gSum += g;</div>
<div><span> </span>bSum += b;</div>
<div><span> </span>}</div>
<div><span> </span>if ( aSum > 255 )</div>
<div><span> </span>aSum = 255;</div>
<div><span> </span>if ( rSum > 255 )</div>
<div><span> </span>rSum = 255;</div>
<div>
<span> </span>if ( gSum > 255 )</div>
<div><span> </span>gSum = 255;</div>
<div><span> </span>if ( bSum > 255 )</div>
<div><span> </span>bSum = 255;</div>
<div><span> </span>targetCursor.get().set(
ARGBType.rgba( rSum, gSum, bSum, aSum ) );</div>
<div><br>
</div>
<div>I have an ImgPlus backed by an RGB PlanarImg of
UnsignedByteType and ARGBType.alpha(value) is 255 for all of
them, so aSum is 765. It would appear that the correct
solution would be to divide aSum by 3. In addition, there's no
scaling of the individual red, green and blue values by their
channel's alpha. If the input were two index-color images,
each of which had different alphas, the code should multiply
the r, g and b values by the alphas before summing and then
divide by the total alpha in the end. The alpha in this case
*should* be the sum of alphas divided by the number of
channels.</div>
<div><br>
</div>
<div>However, I think the problem is deeper than that. For an
RGB ImgPlus, there are three LUTs and each of them has an
alpha of 255, but that alpha only applies to one of the colors
in the LUT. When you're compositing images and weighing them
equally, if two are black and one is white, then the result is
1/3 of the white intensity - if you translate that to red,
green and blue images, the resulting intensity will be 1/3 of
that desired. This might sound weird, but the only solution
that works out mathematically is for the defaultLUTs in the
DefaultDatasetView to use color tables that return values that
are 3x those of ColorTables.RED, GREEN and BLUE. Thinking
about it, I'm afraid this *is* the correct model and each
channel really is 3x brighter than possible.</div>
<div><br>
</div>
<div>It took me quite a bit of back and forth to come up with
the above... I hope you all understand what I'm saying and
understand the problem and counter-intuitive solution and have
the patience to follow it. Dscho, if you made it this far -
you're the mathematician, what's your take?</div>
<div><br>
</div>
<div>--Lee</div>
</div>
<br>
<fieldset></fieldset>
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