add SkLerpXfermode

add SkLerpXfermode

BUG=
R=bsalomon@google.com

Committed: https://code.google.com/p/skia/source/detail?r=9229

[reed1, 2013-05-21 20:02:03]

should we write the gpu backend now or later

[bsalomon, 2013-05-21 20:27:11]

lgtm with minor nit.

If you want to take a crack at the gpu version, go ahead. Otherwise, later.

https://codereview.chromium.org/15602003/diff/1/gm/lerpmode.cpp
File gm/lerpmode.cpp (right):

https://codereview.chromium.org/15602003/diff/1/gm/lerpmode.cpp#newcode42
gm/lerpmode.cpp:42: virtual SkString onShortName() {
override (here and below)

[reed1, 2013-05-21 21:33:20]

Committed patchset #2 manually as r9229 (presubmit successful).

[Justin Novosad, 2013-05-22 14:13:37]

My 2 cents: I find "Lerp" is a very hacky name. I think this should be called
"LinearBlend" and the parameter should be called "blendFactor". Other terms that
are often used to describe this effect in compositing jargon are "Crossfade" and
"Dissolve" (except in photoshop where dissolve means something else).

Also, in GLES2 this can be expressed as a blendFunc by setting the constant
alpha and using the blend coefficients constant_alpha and
one_minus_contant_alpha.  With this in mind, I wonder if there would be any
value in adding support for constant color/alpha values in SkXfermode, allowing
this blend mode to be expressed using coefficients...

[reed1, 2013-05-22 14:22:09]

On 2013/05/22 14:13:37, junov wrote:
> My 2 cents: I find "Lerp" is a very hacky name. I think this should be called
> "LinearBlend" and the parameter should be called "blendFactor". Other terms
that
> are often used to describe this effect in compositing jargon are "Crossfade"
and
> "Dissolve" (except in photoshop where dissolve means something else).
> 
> Also, in GLES2 this can be expressed as a blendFunc by setting the constant
> alpha and using the blend coefficients constant_alpha and
> one_minus_contant_alpha.  With this in mind, I wonder if there would be any
> value in adding support for constant color/alpha values in SkXfermode,
allowing
> this blend mode to be expressed using coefficients...

As a graphics API, I don't find lerp or bilerp out-of-bounds for a name (we use
bilerp publicly in GrTextureAccess.h), but if there is a consensus for a
different name, I'm fine w/ that too.

As for how best to communicate this to ganesh, I defer to that team.