PS CS4 randomly saves psd files in 24bit or 32bit

[Elmer B]

This one has got me slightly (only slightly? !) confused.

I have a collection of multi-layered psd files which contain on average 10 saved layer comps used for creating system icons. In total there are 66 files, looking at their thumbnails in Windows 7 the other day I noticed that some had a "white" or blank background (not included in any of the images) but 31 of the thumbnails have a chequered background (aka the Photoshop CS4 screen). On trying to find out why I discovered that the white/blank background are 24 bit psd images/files (8 bits per channel). Whereas the ones with the chequerboard pattern are 32 bit psd images/files (8 bits per channel).

Question is, how the heck did I manage this ? I've not yet been able to recreate a 32 bit (8 bits per channel) psd file. None of the 32 bit psd files has a visible alpha channel in the channels column, but I wouldn't expect one as they are multi layered comps with slight variations.

Any guidance greatly appreciated.

By the way system is 64 bit so I'm jumping from 32 bit to 64 bit Photoshop totally randomly, the only time I notice is when I try to use one of my older plug-ins, and there they are. Gone !! (Must be in 64 bit now then, mutter mutter!). Would this possibly have anything to do with it ??

[mistermonday]

Elmer, There are no such things as 24 and 32 bit image files. They are either 8 bit or 16 bit. The terms 24, 36, and 48 bits are used by scanner manufacturers meaning they produce 3 channels whose data are 8, 12, or 16 bit. However that use of 24/36/48 is really not appropriate and I think originated as a marketing gimmick. When an image file is opened in a graphics program such as Photoshop, they are handled as either 8 bit or 16 bit data only.
If you have converted your RGB image to CMYK, then you will have 4 channels. By your definition that may be 32 bits but it is really an 8bit image with 4 color channels.
Regards, Murray

[Elmer B]

Thanks for the reply Murray.

Yes, I fully understand that the images are 8 bits per channel or 16 bits per channel, especially after spending hours on an image only to discover Photoshop was in "16 bit mode" . What I would like to know is why / how I have some psd files with Windows 7 telling me that some have a bit depth* of 24 (8+8+8) while others have a bit depth* of 32 (8+8+8+[I]8(?)[/8]).

*Change of terminology there. Sans doute, that'll change everything !!

I only (well, 90%+ of the time) work with RGB. I only ever work with 8 bits per channel. I have added alpha masks in the past to get RGB/A or bit depth of 32. But in these cases no I haven't as they will be saved as PNG files which adds the Alpha channel or transparency anyway. Or they are converted to icons. I've attached two psd files to better show what I'm talking about. There is also a PNG snapshot of my folder thumbnails for the side by side comparison view.

Regards

Elmer B

[Doug Nelson]

Windows uses 8bits to handle the transparency of thumbnails. It's reporting 32bit (8 each for RGB and 8 for transparency) for the thumbnail image, not for the file itself.

[Elmer B]

I can accept that, but why do some of the thumbnails report 24 bit ? (I'm a pain I know!!). It's just one of those things in life that niggles at ya until you fully understand, then it's like a lightbulb going on !