Hi to all,
This is a newbie question related to color image processing ...
I've got a lot of computer vision and image processing algorithms that
works on greyscale images, i.e. 8-bit, 16-bit and float greyscale images.
I want to update my algorithms and process color images.
There are very few research papers on color image processing and each
paper use a different method to process such images (e.g. a different
color space, a different white point, ...).
While I'm well versed in computer vision (using greyscale images), I'm a
newbie to color image processing, and the more I read research papers or
books or FAQ or web articles on the subject, the more I am confused
about all these conflicting informations and different mathematics ...
Nevertheless, to sum up what I have learned, this is what I need to do
to convert my algorithms :
1) First, I want to deal with RGB-24-bit, RGB-48-bit and RGB float (also
known as HDR) images
2) To process such images, I need to convert them to Yxy or CIE Lab
color space
3) Then, I need to apply my greyscale algorithms to the Y (Yxy) or to
the L (CIE Lab) component
4) I then have to convert back the transformed image to a RGB color
space (RGB 24-/48-bit or RGB float)
5) for HDR images, once I have a transformed RGB float image, I can
apply a tone mapping algorithm and convert to 48-bit (or 24-bit)
The problems I have are the following :
---------------------------------------
A) If I need to write a RGB2XYZ matrix to convert my images, what matrix
should I use ?
My images are taken from unknown files (LDR, HDR float) or from unknown
camera (JPEG 24-bit or RAW converted to 48-bit, no profile attached), so
do I have to assume they have linear RGB color values or do I have to
assume that they use a sRGB color model or do I need to apply a gamma
correction before the transformation ? In the second case (sRGB) or
third case (gamma correction), the transformation matrix is far more
complex to calculate (from a computationnal standpoint). Is this
computation really needed ?
For example, given this link:
http://www.brucelindbloom.com/index.html?Eqn_RGB_XYZ_Matrix.html, and
given a RGB file that is not known, do I need to compute these complex
transformations before using a RGB2XYZ matrix ?
=> And what reference white point should I use (e.g. D50, D65) ???
B) Instead of writing my own transformation matrices, does LittleCMS can
do this for me ?
That would mean: converting from RGB 24-/48-bit and RGB float to a float
color space such as Yxy ?
I noticed that LittleCMS has 16-bit XYZ/Yxy/Lab types and even 8-bit
Lab. How is this possible ??? All research papers I could read work with
float images (not double images and even less 8-bit, and this is
something I can easily understand), so how can you represent such color
spaces as 8-bit (even as 16-bit) ?
16-bit is probably correct for images coming from a digital camera, due
to the limitation of the CCD captors, but what about computer generated
images (i.e. most of the HDR images) ?
I know this is a lot of questions, but as I have said, I'm a newbie to
the field of color image processing and I just want to apply greyscale
image processing algorithms to a color image. Is this really possible ?
Is there a *standard way* or at least a *commonly accepted way* to do
this ?
Thanks in advance for any info or pointers you could give me :)
Hervé Drolon
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