O2D:...

plugin/idl/layer.idl

Index: plugin/idl/layer.idl
===================================================================
--- plugin/idl/layer.idl	(revision 71475)
+++ plugin/idl/layer.idl	(working copy)
@@ -39,13 +39,41 @@
 %]
 [nocpp, include="core/cross/cairo/layer.h"] class Layer : ObjectBase {
   %[
+    Available painting operators.
+
+    \var PaintOperator,
+    \li BLEND,  Alpha-blend the Pattern on top of lower Layers based on its
+                alpha channel. (Default)

[fbarchard, 2011/01/19 18:51:20]

preattenuated or unattenuated alpha?

[Tristan Schmelcher 2, 2011/01/20 00:26:16]

I've never heard of preattenuated/unattenuated alpha ... do you mean
premultiplied/unpremultiplied alpha? Cairo uses premultiplied alpha for
everything, but that's just an implementation detail as far as the JS API is
concerned.

[fbarchard, 2011/01/20 18:06:05]

same thing.
preattenuated = premultiplied
unattenuated = unpremultiplied
So Cairo is premultiplied.  But hardware (o3d) tends to not be.  Maybe worth a
note.

[Tristan Schmelcher 2, 2011/01/20 20:43:32]

It's noted in my new CL to convert to premultiplied alpha in texture_cairo.cc.

+    \li BLEND_WITH_TRANSPARENCY,  Like BLEND, but scale the alpha channel down
+                                  based on the alpha property of the Layer as an
+                                  additional fractional transparency.

[fbarchard, 2011/01/19 18:51:20]

is this destination alpha blending?  (aka Disney method)

[Tristan Schmelcher 2, 2011/01/20 00:26:16]

No, just plain old source alpha blending. The _WITH_TRANSPARENCY version just
means that in addition to using the alpha channel of the Pattern we also use the
alpha property of the Layer. So the blend function is basically (1 - (pattern
alpha * layer alpha)).

[fbarchard, 2011/01/20 18:06:05]

Thats basically what I mean by destination alpha.  The alpha in the frame buffer
is used.  But there are different ways to use that alpha... the most common
being disney's method.
Just make sure you get the same results as O3d.

[Tristan Schmelcher 2, 2011/01/20 20:43:32]

No, "layer alpha" above means the alpha property of _this_ Layer object, not the
per-pixel alpha channel of the destination of the paint operation. The
destination's alpha is not used for anything. In fact the destination does not
even have an alpha channel at all.
It does appear to look the same as O3D.

+    \li COPY,  Copy the colour content of the Pattern directly to the
+               destination, ignoring the alpha channel.
+    \li COPY_WITH_FADING,  Like COPY, but fade the colour to black based on the
+                           alpha property of the Layer as an additional
+                           fractional brightness.

[fbarchard, 2011/01/19 18:51:20]

just alpha, or full ARGB shading?

[Tristan Schmelcher 2, 2011/01/20 00:26:16]

I'm not sure what you're referring to here.

[fbarchard, 2011/01/20 18:06:05]

So the alpha in the destination layer is used to multiply by the ARGB of the
source before doing a 'COPY'.  The destinations ARGB is replaced by the 'faded'
image.

[Tristan Schmelcher 2, 2011/01/20 20:43:32]

No, "alpha property of the Layer" means the alpha property of this Layer object,
i.e. the value returned by this->alpha(). That is used to multiply the ARGB of
the source to fade the image, and then the destination's ARGB is replaced with
that faded image.

+  %]
+  enum PaintOperator {
+    BLEND,
+    BLEND_WITH_TRANSPARENCY,
+    COPY,
+    COPY_WITH_FADING
+  };
+
+  %[
     The Pattern used to paint this Layer.
   %]
   [getter, setter] Pattern? pattern;
 
   %[
-    Transparancy of this layer.
+    Whether this layer should be visible or not.
   %]
+  [getter, setter] bool visible;
+
+  %[
+    The transparency for the BLEND_WITH_TRANSPARENCY operator or the fading for
+    the COPY_WITH_FADING operator.
+  %]
   [getter, setter] double alpha;
 
   %[
@@ -82,6 +110,11 @@
     A scaling factor to apply to the pattern's y-axis.
   %]
   [getter, setter] double scale_y;
+
+  %[
+    The paint operator to use for painting this Layer.
+  %]
+  [getter, setter] PaintOperator paint_operator;
 };  // Layer
 
 }  // namespace o2d