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timothyn90696161
timothyn90696161
Participant
January 3, 2019
Question

Saving 32bit colour JPEG as 8bit greyscale JPEG but file memory not reduced?

  • January 3, 2019
  • 2 replies
  • 1148 views

Hi,

I wish to save a 32bit colour JPEG image as 8bit greyscale JPEG to reduce memory occupied by image (not worried abut loss of colour).

When I save image using "Image", "mode" menus in Photoshop and then select "greyscale", "8bit per channel" the image is altered to greyscale but on saving the image the file size is not reduced.

I have deliberately saved with quality as maximum and as a JPEG file

Am I doing something wrong?

I would have though reducing the bits per channel from 32bits to 8bits would reduce the overall file size.

Maybe there is another way to achieve my goal?

Many thanks,

tim.

    This topic has been closed for replies.

    2 replies

    T
    Test Screen Name
    Legend
    January 3, 2019

    Another thing to consider is that it may originally have been better compressed: you've chosen high quality, which will compress fairly poorly. You aren't really comparing like with like and actually can't.

    davescm
    Community Expert
    davescmCommunity Expert
    Community Expert
    January 3, 2019

    Hi

    I am puzzled about your first line "32 bits per channel jpeg".  All jpegs are 8 bits/channel.

    So a color jpeg will use 8 bits x 3. In the jpeg file that is converted and stored as Y Cb Cr and the colour components are compressed more than the luminance.
    The jpeg format does allow for a single channel but many apps just save as three but of the colour components, containing no info, compresses completely.

    Saving a grayscale from Photoshop does reduce file size but remember the jpeg is compressed more on the colour channels so the reduction will not be to a third.

    I've just checked with a sample colour image. Saving as RGB colour saved at 8.6MB filesize and the grey at 5.9MB filesize (both at quality 12)

    Dave

    D Fosse
    Community Expert
    D FosseCommunity Expert
    Community Expert
    January 3, 2019

    I have a feeling this is "ancestors metadata" again. That's a section in metadata recording file history, and in some cases, especially with a history of many copy/paste operations, this metadata can accumulate to many MB. IOW, the bulk of the file size is in metadata, not the image data.

    Save For Web / Export will strip out this metadata, but a regular Save As will not. Another ACP, Stephen A Marsh, has a script to remove it. He should get a notification with this post if I did this right

    davescm
    Community Expert
    davescmCommunity Expert
    Community Expert
    January 3, 2019

    The other possibility is that the image in RGB mode had little colour info. If so, converting fully to grey then saving will have minimal impact on the filesize.

    Dave

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