Computer Graphics with Photoshop - reducing images and keeping quality going from large CMYK jpg to small web jpg

Author

reducing images and keeping quality going from large CMYK jpg to small web jpg

windandwaves

2007-06-14, 6:15 pm

Hi Folk

If I would like to reduce images while keeping quality as good as it
can be - going from large CMYK jpg images (e.g. 20 Megabyte) to small
jpg images for the web (10kb) then what steps should I take? Should i
just reduce and save for the web (high quality) or do I also need to
convert to RGB, choose specific reduction methods, etc....

Thanks in advance

Nicolaas

Mike Russell

2007-06-14, 10:14 pm

"windandwaves" <nfrancken@XXXXXXXXXX> wrote in message
news:1181858583.493565.306310@q19g2000prn.googlegroups.com...
> Hi Folk
>
> If I would like to reduce images while keeping quality as good as it
> can be - going from large CMYK jpg images (e.g. 20 Megabyte) to small
> jpg images for the web (10kb) then what steps should I take? Should i
> just reduce and save for the web (high quality) or do I also need to
> convert to RGB, choose specific reduction methods, etc....

CMYK images have less contrast and saturation than their RGB counterparts,
so there is no problem with maintaining the quality when making this
conversion. In general you will get a better looking image by converting to
either sRGB or Lab and bumping color and contrast to cover the full nominal
range of black to white. Particular images may benefit from special
treatment - for example it's easy to boost green foliage in Lab. Then, if
necessary, convert to sRGB, and save for web.

If time is limited, for example if you have 100's of these to do in a short
time, run auto-color on each image before saving, or auto-levels and
auto-color and pick the best of the two.

Sharpening is another important part of conversion, particularly for
relatively small images. Generally Photoshop's Unsharp Mask has enough
flexibility to get very good quality. If you are using Lab, sharpening is
best done in that color space, on the Lightness channel only.

Compression works. If you are limited by file size, and not the pixel
dimensions, then for a given file size you will get a better result with a
lower jpeg quality setting, and a larger image.
--
Mike Russell - www.curvemeister.com

tacit

2007-06-14, 10:14 pm

In article <1181858583.493565.306310@q19g2000prn.googlegroups.com>,
windandwaves <nfrancken@XXXXXXXXXX> wrote:

> If I would like to reduce images while keeping quality as good as it
> can be - going from large CMYK jpg images (e.g. 20 Megabyte) to small
> jpg images for the web (10kb) then what steps should I take? Should i
> just reduce and save for the web (high quality) or do I also need to
> convert to RGB, choose specific reduction methods, etc....

Save for Web automatically converts to RGB, because Web browsers can't
read CMYK JPEGs.

--
Photography, kink, polyamory, shareware, and more: all at
http://www.xeromag.com/franklin.html

tacit

2007-06-15, 3:19 am

In article <H8kci.29606$JZ3.17597@newssvr13.news.prodigy.net>,
"Mike Russell" <RE-MOVEmike@Curvemeister.comRE-MOVE> wrote:

> CMYK images have less contrast and saturation than their RGB counterparts,
> so there is no problem with maintaining the quality when making this
> conversion.

Untrue--or at least, misleading.

CMYK does not offer as much saturation in some colors, notably blues. In
some other colors, such as yellow, CMYK allows *more* saturation than
RGB.

CMYK's range of contrast is greater than RGB's, in part because you're
working with a black channel, which allows for a great deal of contrast
and shadow detail.

--
Photography, kink, polyamory, shareware, and more: all at
http://www.xeromag.com/franklin.html

Johan W. Elzenga

2007-06-15, 6:14 pm

tacit <tacitr@aol.com> wrote:

> In article <H8kci.29606$JZ3.17597@newssvr13.news.prodigy.net>,
> "Mike Russell" <RE-MOVEmike@Curvemeister.comRE-MOVE> wrote:
>
>
> Untrue--or at least, misleading.
>
> CMYK does not offer as much saturation in some colors, notably blues. In
> some other colors, such as yellow, CMYK allows *more* saturation than
> RGB.

Actually, it's even more complicated than that. You and Mike are talking
about 'RGB' and 'CMYK' as if they are color *spaces*, but they are not.
RGB and CMYK are color *models*, and you have to specify a specific
color *space* within that model before you can make any meaningful
comparison. Some RGB color spaces, such as ProPhotoRGB, allow more
saturated yellow than almost any CMYK color space. Of course, if you
compare different CMYK spaces with sRGB specifically (because the images
are destined for the web), the remark about yellow is quite true.

--
Johan W. Elzenga johan<<at>>johanfoto.nl
Editor / Photographer http://www.johanfoto.com

windandwaves

2007-06-18, 6:15 pm

thank you all for your answers, I will look into it!