-------- Original Message --------
Jeff - Thanks for the explainations. My goal is to manipulate only the
brightness or luminosity
as effeciently as possible. I'm not very familiar with non-RGB
colorspaces, but I can find out.
Since conversions to HSV and HSL do not exist in PIL, I was hoping to
manipulate the K values
in the CMYK colorspace and then convert back to RGB. I know know that
this is not
possible since this conversion is not done correctly. I could convert
to another colorspace
using numeric or numarray, but this would be slow.
How can the im.point() function be used for an algorithm that requires
"If" statements ?
The "im.point
(table)" variation is
undocumented and the "im.point
(function) image"
is variation is limited to producing only binary mode "1" images. What
is that token "image"
doing at the end there ? Not only that, but the existing documentation
must be wrong, since
both variations call for a "mode" parameter that is not used at all in
the documentation's own example !
Do you know what's really going on ?
Thanks,
Ray Pasco
Jeff Epler wrote:
The algorithm that im.convert("CMYK") uses is this: (C code)
static void
rgb2cmyk(UINT8* out, const UINT8* in, int xsize)
{
int x;
for (x = 0; x < xsize; x++) {
/* Note: no undercolour removal */
*out++ = ~(*in++);
*out++ = ~(*in++);
*out++ = ~(*in++);
*out++ = 0; in++;
}
}
All numbers are in the inclusive range [0, 255] in this case. If "in"
was 0, then out is 255, so it's like the algorithm on the page you
mentouned for RGB->CMY, where the value 255 corresponds to 1.0, and K is
simply set to 0 all the time.
Using numeric or numarray, it is probably possible to do the CMY->CMYK
conversion step described on the page you mentioned, and have the
resulting code run fairly quickly.
As for conversions from RGB to other 3-component color systems, you can
use the "im.convert(mode, matrix)" function with the right matrix. An
example is in the documentation for converting to the XYZ colorspace.
This block of code, common to many of the conversion functions on that
web page you mentioned, has to do with doing gamma conversion:
if ( var_R > 0.0031308 ) var_R = 1.055 * ( var_R ^ ( 1 / 2.4 ) ) - 0.055
else var_R = 12.92 * var_R
if ( var_G > 0.0031308 ) var_G = 1.055 * ( var_G ^ ( 1 / 2.4 ) ) - 0.055
else var_G = 12.92 * var_G
if ( var_B > 0.0031308 ) var_B = 1.055 * ( var_B ^ ( 1 / 2.4 ) ) - 0.055
else var_B = 12.92 * var_B
To perform this step in PIL, you would use the im.point method with a
properly initialized table.
Jeff
|
_______________________________________________
Image-SIG maillist - [email protected]
http://mail.python.org/mailman/listinfo/image-sig
