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+<script type="text/javascript"
+  src="http://cdn.mathjax.org/mathjax/latest/MathJax.js?config=TeX-AMS-MML_HTMLorMML">
+</script>In the Porter/Duff compositing algebra, images are equipped with an alpha channel that determines on a per-pixel basis whether the image is there or not. When the alpha channel is 1, the image is fully there, when it is 0, the image isn't there at all. When it is in between, the image is partially there. In other words, the alpha channel describes the the shape of the image, it does not describe opacity. The way to think of images with an alpha channel is as irregularly shaped pieces of cardboard, not as colored glass. Consider these two images:
+<blockquote><img src="wp-content/uploads/2013/03/source.png">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<img src="wp-content/uploads/2013/03/dest.png"></blockquote>
+
+When we combine them, each pixel of the result can be divided into four regions:<blockquote><img src="wp-content/uploads/2013/03/diagram.png"></blockquote>
+
+One region where only the source is present, one where only the destination is present, one where both are present, and one where neither is present.
+
+By deciding on what happens in each of the four regions, various effects can be generated. For example, if the destination-only region is treated as blank, the source-only region is filled with the source color, and the 'both' region is filled with the destination color like this:
+<blockquote><img src="wp-content/uploads/2013/03/destatop-diagram.png"></blockquote>
+The effect is as if the destination image is trimmed to match the source image, and then held up in front of it:
+<blockquote><img src="wp-content/uploads/2013/03/destatop.png"></blockquote>
+The Porter/Duff operator that does this is called "Dest Atop".
+
+There are twelve of these operators, each one characterized by its behavior in the three regions: source, destination and both (the 'neither' region is always blank). The source-only region can be either 'source' or 'blank'. The destination-only region can be either 'destination' or 'blank'. The 'both' region can be either 'blank', 'source', or 'destination'.
+
+The formula for the operators is a linear combination of the contents of the four regions, where the weights are the areas of each region:
+<blockquote>\(A_\text{src} \cdot [s] + A_\text{dest} \cdot [d] + A_\text{both} \cdot [b]\)</blockquote>
+
+Where \([s]\) is either 0 or the color of the source pixel, \([d]\) either 0 or the color of the destination pixel, and \([b]\) is either 0, the color of the source pixel, or the color of the destination pixel. The areas are given by these formulas:<blockquote>
+\(A_\text{src} = \alpha_\text{s} \cdot (1 - \alpha_\text{d})\)
+\(A_\text{dst} = \alpha_\text{d} \cdot (1 - \alpha_\text{s})\)
+\(A_\text{both} = \alpha_\text{s} \cdot \alpha_\text{d}\)
+</blockquote>
+
+The alpha channel of the result is computed in a similar way. There is
+full coverage in all regions that are not blank:
+<blockquote>
+\(A_\text{src} \cdot [\text{as}] + A_\text{dest} \cdot [\text{ad}] + A_\text{both} \cdot [\text{ab}]\)
+</blockquote>
+where \([\text{as}]\) and \([\text{ad}]\) are either 0 or 1 depending on whether the source and destination regions are present, and where \([\text{ab}]\) is 0 when the 'both' region is blank, and 1 otherwise.
+
+Here is a table of all the Porter/Duff operators:
+<blockquote><table>
+<tr><td/><td>\([\text{s}]\)</td><td>\([\text{d}]\)</td><td>\([\text{b}]\)</td></tr>
+<tr><td>Src</td><td>\(s\)</td><td>\(0\)</td><td>s</td></tr>
+<tr><td>Atop</td><td>\(0\)</td><td>\(d\)</td><td>s</td></tr>
+<tr><td>Over</td><td>\(s\)</td><td>\(d\)</td><td>s</td></tr>
+<tr><td>In</td><td>\(0\)</td><td>\(0\)</td><td>s</td></tr>
+<tr><td>Out</td><td>\(s\)</td><td>\(0\)</td><td>\(0\)</td></tr>
+<tr><td>Dest</td><td>\(0\)</td><td>\(d\)</td><td>d</td></tr>
+<tr><td>DestAtop</td><td>\(s\)</td><td>\(0\)</td><td>d</td></tr>
+<tr><td>DestOver</td><td>\(s\)</td><td>\(d\)</td><td>d</td></tr>
+<tr><td>DestIn</td><td>\(0\)</td><td>\(0\)</td><td>d</td></tr>
+<tr><td>DestOut</td><td>\(0\)</td><td>\(d\)</td><td>\(0\)</td></tr>
+<tr><td>Clear</td><td>\(0\)</td><td>\(0\)</td><td>\(0\)</td></tr>
+<tr><td>Xor</td><td>\(s\)</td><td>\(d\)</td><td>\(0\)</td></tr>
+</table></blockquote>
+And here is how they look:
+<blockquote><img src="wp-content/uploads/2013/03/table.png"/></blockquote>
+It is important to understand that despite being referred to as alpha
+blending, and despite alpha often being used to model opacity,
+conceptually Porter/Duff is not a way to blend the source and
+destination shapes. It is instead a way to overlay, combine and cut
+them as if they were pieces of cardboard. The only places where source
+and destination pixels are actually blended is where the antialiased
+edges meet.
+
+<b>Blending</b>
+Photoshop and the Gimp have the concept of layers which are images stacked on top of each other. In Porter/Duff, stacking images on top of each other is done with the "Over" operator, which is also what Photoshop/Gimp use by default to composite layers:
+<blockquote><img src="wp-content/uploads/2013/03/over-diagram.png"></blockquote>
+Conceptually, two pieces of cardboard are held up with one in front of the other. Neither shape is trimmed, and in places where both are present, only the top layer is visible.
+
+A layer in these programs also has an associated <em>Blend Mode</em> which can be used to modify what happens in places where both are visible. For example, the 'Color Dodge' blend mode computes a mix of source and destination according to this formula:<blockquote>
+\(\begin{equation*}
+B(s,d)= 
+\begin{cases} 0 & \text{if \(d=0\),}
+\\
+1 & \text{if \(d \ge (1 - s)\),}
+\\
+d / (1 - s) & \text{otherwise}
+\end{cases}
+\end{equation*}\)
+</blockquote>
+
+The pixel diagram is this:
+<blockquote><img src="wp-content/uploads/2013/03/colordodge-diagram.png"></blockquote>
+And the result looks like this:
+<blockquote><img src="wp-content/uploads/2013/03/colordodge.png"></blockquote>
+
+Unlike with the regular Over operator, in this case there is substantial chunk of the output where the result is actually a mix of the source and destination.
+
+Layers in Photoshop and Gimp are not tailored to each other (except for layer masks, which we will ignore here), so the compositing of the layer stack is done with the source-only and destination-only region set to source and destination respectively. However, there is nothing in principle stopping us from setting the source-only and destination-only regions to blank, but keep the blend mode in the 'both' region so that tailoring could be supported alongside blending. For example, we could set the 'source' region to blank, the 'destination' region to destination, and the 'both' region to ColorDodge:
+<blockquote><img src="wp-content/uploads/2013/03/colordodge-dest-diagram.png"/></blockquote>
+The result looks like this:
+<blockquote><img src="wp-content/uploads/2013/03/colordodge-dest.png"></blockquote>
+Here are all the possibilities that involve a ColorDodge blend mode:
+<blockquote>
+<img src="wp-content/uploads/2013/03/colordodge-none.png"/>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<img src="wp-content/uploads/2013/03/colordodge-source.png"/>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<img src="wp-content/uploads/2013/03/colordodge-dest.png"/>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<img src="wp-content/uploads/2013/03/colordodge-both.png"/></blockquote>
+With three trivial blend modes:
+<table>
+<tr><td>Source:</td><td>\(B(s, d) = s\)</td></tr>
+<tr><td>Dest:</td><td>\(B(s, d) = d\)</td></tr>
+<tr><td>Zero:</td><td>\(B(s, d) = 0\)</td></tr>
+</table>
+we can generate all of the twelve original Porter/Duff operators.
+
+Reinterpreting Porter/Duff in this way leads to a natural generalization where the blend mode can be chosen from a large set of formulas, where each such formula gives rise to four new compositing operators characterized by whether the source and destination are blank or contain the corresponding pixel color.
+
+The general formula is still an area weighted average:
+<blockquote>\(A_\text{src} \cdot [s] + A_\text{dest} \cdot [d] + A_\text{both}\cdot B(s, d)\)</blockquote>
+where [s] and [d] are the source and destination colors respectively or 0, but where \(B(s, d)\) is no longer restricted to one of \(0\), \(s\), and \(d\), but can instead be chosen from a large set of formulas.
+
+In this way, each new blend mode gives rise to four new compositing operators characterized by whether or the source and destination regions are present or blank.
+
+The output of the alpha channel is the same as before:
+<blockquote>\(A_\text{src} \cdot [as] + A_\text{dest} \cdot [ad] + A_\text{both} \cdot [\text{ab}]\)</blockquote>
+except that [ab] is now determined by the blend mode. For the Zero blend mode there is no coverage in the both region, so [ab] is 0; for most others, there is full coverage, so [ab] is 1.
+
+Here is a table of the operators that are generated by various blend modes:
+<blockquote><img src="wp-content/uploads/2013/03/colordodge-table.png"></blockquote>
+The three first rows are the original Porter/Duff operators.
+
+
+<!--
+
+Alpha Channels
+
+
+The resulting value corresponds to the amount of light reflected by
+the covered part of the pixel, so in order to get the right color
+value, we have to divide by alpha. Since divisions are expensive and
+since source and destination inputs have to be multiplied with their
+corresponding alpha values anyway, pixels are often stored in a format
+where all the color channels have been multiplied with the alpha
+channel.
+
+With this format, the formula becomes:
+
+    (1 - ad) * [s] + (1 - as) * [d] + as * ad * [b]
+
+where [s] and [d] are the premultiplied source and destination colors,
+and [b] is either 0, or one of the premultiplied source or destination
+colors divided by their corresponding alpha channel (this division is
+then canceled by either the /as/ or the /ad/ in front of [b]).
+
+The alpha channel is computed with the same formula. In this case [s]
+is either 0 or as, [d] is either 0 or ad, and [b] is either 0, or
+as/as = 1, or ad/ad = 1.
+
+In other words, with premultiplied pixels, this formula:
+
+   (1 - ad) * [s] + (1 - as) * [d] + as * ad * [b]
+
+works for all four channels of a premultiplied pixel.
+
+
+
+With the classic Porter/Duff operators the output alpha channel is
+computed by plugging 1 into the area weighted average whenever a
+region is filled with either source or destination, and 0 whenever a
+region is blank. We can also view this as plugging in 1, 1 in the
+blend mode formula:
+
+      Source:	B(s, d) = s;	B(1, 1) = 1
+      Dest:	B(s, d) = d;	B(1, 1) = 1
+      Zero:	B(s, d) = 0;	B(1, 1) = 0
+
+Most of the new blend mode formulas satisfy B(1, 1) = 1, which means
+it is tempting to define the alpha computation to be exactly the same
+as the color computation, just as it is for the Porter/Duff
+operators. As long as B(1, 1) = 1, we get:
+
+    ar = (1 - ad) * as + (1 - as) * ad + as * ad * B(as/as, ad/ad)
+       = (1 - ad) * as + (1 - as) * ad + as * ad * B(1, 1)
+
+A very nice result of this is that the ADD operator that simply adds
+all four channels can be defined in this framework with the blend
+mode:
+
+	Plus:	B(s,d) = s + d
+
+When plugging in 1, 1, you get:
+
+     ar = (1 - ad) * as + (1 - as) * ad + as * ad * B(1, 1)
+        = (as - ad * as + ad - as * ad + as * ad * (1 + 1)
+	= as + ad
+
+and similarly for the color channels:
+
+     r = (1 - ad) * s + (1 - as) * d + as * ad * (s/as + d/ad)
+       = s - s * ad + d - as * d + ad * s + as * d
+       = s + d
+
+Unfortunately, there are two annoying exceptions: The blend modes
+Exclusion and Difference do not produce 1 when you plug in (s = 1, d =
+1), they produce 0. There is some logic to that, because these blend
+modes are concerned with the difference between source and
+destination, and the difference in coverage in the 'both' region
+(where source and destination are both present by definition) would in
+fact be zero. Nonetheless, making the alpha channel 0 in the 'both'
+region would mean that you couldn't see the blended result, so that's
+not a useful option.
+
+The right solution may be instead to define inverted versions:
+
+    Inverted Exclusion:		B(s,d) = 1 - Exclusion(s,d)
+				       = 1 - s - d + 2 * s * d
+
+    Inverted Difference:	B(s,d) = 1 - Difference(s,d)
+    	     			       = 1 - |s - d|
+
+These will produce the correct alpha channel, and users can still
+achieve the non-inverted blend modes by using the inverted blend modes
+twice.
+
+Finally, there are four "non-separable" blend modes that don't operate
+on a per-channel basis. These instead conceptually convert the source
+and destination into the HSL representation and then for each of the
+H, S, L channels pick either the source or the destination
+value. There is obviously no way to unify this operation into one
+formula that applies to all four channels.
+
+
+Conclusion
+
+It is possible to generalize the Porter/Duff compositing algebra by
+allowing more behaviors in the 'both' region of the Porter/Duff
+pixel. If the effect on the alpha channel is defined to be B(1,1),
+then all four channels can be computed with the same formula:
+
+  (1 - ad) * [s] * S + (1 - as) * [d] * D + as * ad * B(S/as, D/ad)
+
+where [s] and [d] are 1 or 0 depending on whether the source and
+destination are drawn, and B is the blend mode. When B(s,d) is chosen
+to be 0, s, or d, the formula reduces to the twelve original
+Porter/Duff operators.
+
+-->
+
+<!--
+
+CSS compositing spec
+
+There is a proposal for extending CSS
+[https://dvcs.w3.org/hg/FXTF/rawfile/tip/compositing/index.html] with
+support for blending and compositing that also tries to unify
+porter/duff compositing with blend modes. In this scheme a Porter/Duff
+operator and a blend mode are chosen orthogonally.
+
+For a given pixel, the source and the destination are blended
+according to the chosen blendmode, and then a linear interpolation
+between the source and the blended result is computed with the
+destination alpha as the interpolation weight. This result is then
+used as the source in compositing with the chosen Porter/Duff
+operator. The full formula is this:
+
+      Fa * as * [ (1 - ad) * Cs + ad * B(s,d) ] + Fb * ad * Cd
+
+where Fa and Fb are weight factors corresponding to the chosen
+Porter/Duff operator.
+
+This formula has some useful properties: If the blend mode is chosen
+to be Normal, that is, B(s,d) = Cs, then the formula becomes identical
+to whatever Porter/Duff operator is involved. If the Porter/Duff
+operator is chosen to be Over, then the formula becomes identical to
+the Photoshop/Gimp layer blending. When the operator is Source, the
+formula produces the atop-like result shown above.
+
+However, there is no conceptual underlying reason that the formula has
+these properties. You can't give a graphical interpretation of it the
+way you can with Porter/Duff; the useful properties the formula has
+might as well be numerological accidents. In fact, for many operators,
+the formula produces nonsensical results.
+
+For example, here is what it looks like when the operator is DEST_OVER
+and the blend mode is ColorDodge:
+
+    DEST_OVER / COLOR_DODGE
+
+What is this? It's DEST_OVER, except that the antialiasing is messed
+up with a weird purple shadow. There is no situation where this is the
+operation you need.
+
+There is also a bunch of operators where the blend mode is simply
+ignored: DEST, DEST_IN, DEST_OUT, CLEAR.
+
+The fundamental problem is that it doesn't make any sense to select a
+Porter/Duff compositing operators and a blend mode orthogonally. The
+Porter/Duff operator and the blend mode both specify what happens in
+the region where source and destination overlaps, and if they disagree
+on what happens there, it is not clear what happens. The answer to
+this is not to pick some formula that happens to produce useful
+results in certain special cases.
+
+-->