[whatwg] several messages

Philip Taylor excors+whatwg at gmail.com
Sun May 20 09:53:13 PDT 2007

On 20/05/07, Mathieu HENRI <p01 at opera.com> wrote:
> Ian Hickson wrote:
> > I've referenced the paper and dropped 'darker'.
> Please, please, pretty please, bring 'darker' back.
> Rename it 'multiply' if you want, or if like me you think this name
> better reflects the operation previously known as 'darker'.

I think this is more like an entirely new operation, rather than being
related to 'darker', given that 'darker' has three totally different
implementations and none of them multiply. In the cases where all the
alpha values are 1, and when CO is the non-premultiplied output colour
component and CA and CB are the input (source and destination)
colours, I believe you get:

Mozilla: CO = CB
Opera: CO = min(CA, CB)
Safari: CO = CA + CB - 1

If you care about alpha, and take cX as premultiplied colour
components (and CX as non-premul, and aX as alpha), then you get:
  cO = cA * min(1, (1-aB)/aA) + cB
  aO = aA * min(1, (1-aB)/aA) + aB
  CO = min( (1-aA)*CB + CA, (1-aB)*CA + CB )
  aO = 1 - (1-aA)*(1-aB)
  CO = 1 - (1-CA)*aA - (1-CB)*aB  except not quite
  aO = aA + aB

(Opera is kind of similar to SVG 1.1's 'darker', except not the same
since it's doing it on non-premultiplied colours instead.)

> Here is a list of use cases for 'darker', I already applied in realtime
> animations:
> 1. filtering and/or separating the channels of a canvas *very* quickly.
> 2. multiplying images. This can be really useful to compose/combine images.
> 3. "burning" images. To do so, copy the canvas A onto a smaller canvas
> B, fill B with a white rectangle of medium/high opacity if you want,
> then draw B on top of A in 'daker'. Tada, you just enhanced the dark
> areas of A and even introduced some nice color bleeding.

If there was a 'multiply', it should probably be defined as something like:
  CO = CA * CB
  aO = aA * aB
(taking all values to be in the range [0,1]) because that actually
makes some sense and it seems to be useful.

For case 1, you could multiply by (1, 0, 0, 1) to extract the R
channel. (You wouldn't be able to combine the channels to make
greyscale, but you can't do that now anyway - that'd need something
like OpenGL's dot operator, which does RO=GO=BO = RA*RB + GA*GB +
BA*BB, which would probably be a nice one to add if we wanted more

For case 2, I'm not sure why you can't do that already. You can reduce
the brightness by drawing a (0, 0, 0, 0.5) rectangle on top of the
image. If it's e.g. a spotlight where you want to multiply by an image
rather than a solid colour, you can make that image be black and store
its shape in its alpha channel, then draw that on top with source-over
to mask out certain areas. You can make an image brighter by drawing
on top of itself with 'lighter' to get up to 2x brightness (and repeat
for exponential brightening). Are there specific situations which
can't already be handled without 'darker'?

For case 3, I think 'multiply' above would possibly handle that,
depending on exactly what the desired effect is.

I assume 'multiply' would be a bit more complicated to implement and
less efficient than the other operations, since the graphics libraries
usually don't provide that function; but maybe that's acceptable. At
least the definition above is identical whether you store
premultiplied or non-premultiplied components, so it'll only ever be a
multiplication and (I think) a couple of shifts and an addition per
component, which seems fast enough.

Maybe the new 'multiply' operation should just be named 'darker' so
it's compatible with old browsers (though very buggily implemented in
all of them), but defined like above.

(Just for completeness with respect to my original email about
composite operations, Opera's implementation of 'lighter' appears to
  if aA == 0:
    CO = CB
    aO = aB
  else if aB == 0:
    CO = CA
    aO = aA
    CO = max(CA, CB)
    aO = 1 - (1-aA)*(1-aB)
which has very odd behaviour, since the output changes drastically if
you change alpha from 0.0 to 0.01. In all the implementations,
'lighter' is confusingly unlike 'darker' - names like 'plus' and
'multiply' make much more sense.)

Philip Taylor
excors at gmail.com

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