Re: Luminance and RGB layers question

Martin Leese <please@xxxxxxxxxxxxxxxxxxxxxxxxxx> writes:

>Just to add a little bit to this excellent response.
>Some of the colour spaces you could use for this
>task are HSI, HSV, Lab. Delving into these might
>help you. The general procedure is to convert your
>RBG image to, say, HSI. Blend your graytone image
>into the Intensity (or Value or Luminance), then
>convert back to RGB.

Also, it's important to note that there are two different classes of
"luminance plus colour" colour spaces, with different processing
required if you change luminance.

Some colour spaces have colour components *that change in size along
with luminance*. For example, look at the television YCbCr or YPbPr
luminance plus colour difference representation. When the image gets
brighter (e.g. by opening the camera lens aperture), Y increases but so
do Cb and Cr. In a space like this, if you change the Y value you
*also* have to multiply Cb and Cr by the ratio new_Y/old_Y to keep the
colour the same. (It's actually more complex than this in video, since
zero brightness is not a zero pixel value).

Other colour spaces like L*a*b* have colour channels that are
independent of luminance. With these, you don't need to adjust the
colour data when changing luminance.

Dave
.