Why color converted PSD files get larger?

AW
Posted By
Allen_W
Jan 12, 2004
Views
959
Replies
19
Status
Closed
Why do color converted PSD files get a lot larger in file size (seems to increase by a % of the original file size)? This happens if I convert to another color space or color profile. Is there a way to reduce the file size back to the size of the pre-converted file?
RL
Robert_Levine
Jan 12, 2004
When you say color converted, do you mean from RGB to CMYK? If so, you’re going to get more data in that file immediately by adding the 4th color channel.

Bob
AW
Allen_W
Jan 12, 2004
RGB to CMYK and back to RGB still adds a lot more data. It seems that the file size increase more with each conversion.

BTW: CMYK files are larger than RGB to begin with?
RW
Rene_Walling
Jan 12, 2004
Allen,

Greyscale files have one Channel
RGB files have three
and CMYK have four

Therefore, for the same image size, an RGB file will be three times as big as a greyscale one and a CMYK file will be 1.33 times bigger than the RGB one (or four time the greyscale one)

The file size should not keep increasing as you go. Are you sure you are really looking at the file size?
AW
Allen_W
Jan 12, 2004
I thought RGB has a larger color spectrum so I did not know CMYK (with less colors than RGB) was larger.

File sizes are increasing with color conversions (from RGB to CMYK and back to RGB, etc.). I had a 4 MB file that increased to about 12 with a color conversion. Now I know not do convert color modes if I don’t have to. But I still wonder why this file increase happens.
RW
Rene_Walling
Jan 12, 2004
Again, the file size should not increase.

Where do you get the info for the file size?
AW
Allen_W
Jan 12, 2004
The file size increases, unfortunately. I’m trying to find out why. I see the file size changes in Windows Explorer or the file’s properties.

Try it out yourself:

Make a 300 dpi RGB image. Scribble some colors on a couple layers. Save the file and note the file size on your hard drive. Now open it up and convert the file to CMYK and save. Note file size again. Convert it back to RGB and save. Note how the file size is increasing with each color mode conversion.
RW
Rene_Walling
Jan 12, 2004
Make a 300 dpi RGB image. Scribble some colors on a couple layers. Save the file and note the file size on your hard drive. Now open it up and convert the file to CMYK and save. Note file size again. Convert it back to RGB and save. Note how the file size is increasing with each color mode conversion.

Sorry, everything is fine here.

Check the image size dialog box, that’s what matters, not the file size in the Explorer as that may vary depending on the file format you use. How are you saving your files?
PC
Philo_Calhoun
Jan 12, 2004
I wonder whether you are embedding a icc on your final save. That would increase file size.
L
LenHewitt
Jan 12, 2004
Allen,

If you are saving as .PSD you will be fooling yourself. Every time you convert, some data is destroyed. PSD format compresses files. It is quite possible that following a mode change the compression will be less effective. You can only reliably carry out your test when saving in a non-compressed format such as an uncompressed TIFF.

Here are typical figures for the two formats with conversions:

PSD:

RGB 1,994,555

Converted to CMYK 2,889,535

Converted back to RGB 2,637,337

TIFF

RGB 2,995,840

Converted to CMYK 4,548,332

Converted back to RGB 2,996,444

The size of a file ON DISK and in memory can be two VERY different figures….
AW
Allen_W
Jan 12, 2004

[Sorry, everything is fine here.

Check the image size dialog box, that’s what matters, not the file size in the Explorer as that may vary depending on the file format you use. How are you saving your files?]

Try closing the file after each convert save. Then reopen, convert again and save, etc.

I tested with a 300 dpi letter size file (PSD with no ICC profiles), using five layers with different color 300 size brush scribbles. Then went from RGB to CMYK to RGB (saving, closing, and reopening each time). File size was 36.7 MB RGB, then 70 MB CMYK, then 79.3 MB RGB (if continued conversions: then 99 MB CMYK, then 84.4 MB RGB, then 101 MB CMYK, then 86.4 MB RGB). See how the file size is WAY larger than the original 36.7 MB for essentially the same RGB file after a few conversions? That’s why after finding this out, I’ll never convert my files to different color spaces if I don’t need to! I learned the hard way. Now I’m just trying to find out why this happens (and without warning of the consequences). :\
RW
Rene_Walling
Jan 12, 2004
Check Len’s post about saving files
PC
Philo_Calhoun
Jan 12, 2004
It seems strange that the compression algorithm would not continue to provide approximately equal compression. I wonder whether previews are buried somewhere in there and the problem is like MS Word’s quicksave. Len: did you try "save as"?
AW
Allen_W
Jan 12, 2004
Also, if I take the color converted PSD and save it to TIF, that TIF will be larger than than the TIF of the unconverted PSD.
PC
Philo_Calhoun
Jan 12, 2004
I tried a test image of two coloured circles partially overlapping each other and converted first from RGB>LAB>RGB and a second time RGB>CMYK>RGB. Both of these conversions increased file size. However, looking at the histograms, the original RGB had only two luminosity spikes, but the converted to other space and back had wider luminosity variance (probably because of inaccuracies at edges during the conversion). I tried converting from psd>tiff>psd on the mode converted files and it had no effect of file size (suggesting it is not a junk preview being stored). Interesting.
AW
Allen_W
Jan 12, 2004
I wonder if this problem is documented anywhere. Or if it’s some mystery problem no one has ever bothered to mention. :/
L
LenHewitt
Jan 13, 2004
Allen,

Since it is VERY BAD PRACTICE to change modes repeatedly, it is not something that should affect ANYONE
AW
Allen_W
Jan 13, 2004
Well I was doing it just to check how my RGB image would look in CMYK, etc. I also thought I could use it to convert any out-of-gamut RGB colors to in-gamut CMYK colors. Now knowing what happens, I wouldn’t do this anymore.
DM
dave_milbut
Jan 13, 2004
Allen, there’s a key combo to do just that… cmyk preview.

Ah here it is… Crtl-Y.

see Trevor Morris’ excellent keyboard shortcut cheat sheet: < http://user.fundy.net/morris/downloads/PSCSKeyboardShortcuts .pdf>

Here’s his main site:
<http://user.fundy.net/morris/>
B
Brian
Jan 13, 2004
Allen_W wrote:
I thought RGB has a larger color spectrum so I did not know CMYK (with less colors than RGB) was larger.

CMYK is a smaller color space, but this is due to the properties of the ink not the amount of information in the file. The digital image data contains a fourth channel, thereby making it larger than an RGB (three channel) or greyscale (one channel) image.

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