alpha blending with transparent background

[Marshall Greenblatt]

Hi All,

I'm using skia to render a block with alpha transparency on top of a completely transparent background. I would like the result to keep the source RGB values and only change the alpha value. Instead, the whole ARGB value is blended. This appears to be done in the S32A_Blend_BlitRow32_SSE2 function:

alpha     = 0xbf
src_pixel = 0xffffffffffffffffffffffffffffffff
dst_pixel = 0000000000000000000000000000000000
result    = 0xbfbfbfbfbfbfbfbfbfbfbfbfbfbfbfbf

Instead, I'm seeking the following result:

alpha     = 0xbf
src_pixel = 0xffffffffffffffffffffffffffffffff
dst_pixel = 0000000000000000000000000000000000
result    = 0xbfffffffbfffffffbfffffffbfffffff

Does skia support this type of blending in cases where the backround device is completely transparent? If not, would it be reasonable to add support for it?

Related issues:

http://code.google.com/p/skia/issues/detail?id=547
http://code.google.com/p/chromiumembedded/issues/detail?id=584

Thanks,
Marshall

[liang jian]

Hi, I am not an expert of skia, but I hope my anwser will help you.
Your expectation and the current result is actually the same
color. The difference is: the color result of skia is premultiplied
version of your expected color(which is not premultiplied). I think
skia will always store pixels in premultiplied manner for performance
reason. If you prefer unpremultiplied pixels you have to unpremultiply
the pixels generated by skia manually.

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[Tom Hudson]

On Fri, May 25, 2012 at 5:21 AM, Marshall Greenblatt
<magree...@gmail.com> wrote:
> Hi All,
>
> I'm using skia to render a block with alpha transparency on top of a
> completely transparent background. I would like the result to keep the
> source RGB values and only change the alpha value. Instead, the whole ARGB
> value is blended. This appears to be done in the S32A_Blend_BlitRow32_SSE2
> function:
>
> alpha     = 0xbf
> src_pixel = 0xffffffffffffffffffffffffffffffff
> dst_pixel = 0000000000000000000000000000000000
> result    = 0xbfbfbfbfbfbfbfbfbfbfbfbfbfbfbfbf
>
> Instead, I'm seeking the following result:
>
> alpha     = 0xbf
> src_pixel = 0xffffffffffffffffffffffffffffffff
> dst_pixel = 0000000000000000000000000000000000
> result    = 0xbfffffffbfffffffbfffffffbfffffff
>
> Does skia support this type of blending in cases where the backround device
> is completely transparent? If not, would it be reasonable to add support for
> it?

As Jian Liang said, Skia colors are always stored premultiplied - RGB values are *already* multiplied by alpha. So your RGB components must be <= than A, and 0xbffffff would cause asserts throughout Skia to fail.

Tom

[Marshall Greenblatt]

Thanks, accounting for the premultiplied alpha fixed the problem.
 

Tom

[itai.b...@overwolf.com]

If I understand correctly, the suggestion is to re-multiply the RGB values by 1/alpha to gain the original RGB values.

Did I understand correctly? If I did then the problem with this solution is that it changes the RGB values from the original due to rounding. The problem exaggerates with very small alpha values.

If I did not, then is there another way to un-premultiply the values?

A flag would work best for me, but then - how do I set it from CEF?

Itai Bar-Haim,

Overwolf.com

[Tom Hudson]

On Mon, Mar 10, 2014 at 8:27 AM, <itai.b...@overwolf.com> wrote:

If I understand correctly, the suggestion is to re-multiply the RGB values by 1/alpha to gain the original RGB values.

Did I understand correctly? If I did then the problem with this solution is that it changes the RGB values from the original due to rounding. The problem exaggerates with very small alpha values.

If I did not, then is there another way to un-premultiply the values?

A flag would work best for me, but then - how do I set it from CEF?

Premultiplied alpha loses information. There's no way to exactly reconstruct the original values.

That said, if you look at the commit log for Skia, there's ongoing work to support unpremultiplied images in some circumstances. For example, https://code.google.com/p/chromium/codesearch#chromium/src/third_party/skia/include/core/SkImageInfo.h&q=SkAlphaType&sq=package:chromium&type=cs&l=20 shows there's support for image formats that are unpremultiplied. Last October it was extended into SkBitmap (https://codereview.chromium.org/25275004).

Tom