On Sat, 2009-05-30 at 08:00 -0700, James Cobban wrote:

I scanned some microfilm yesterday. For some reason the software does not support JPEG so I scanned into TIFF format, putting 5 images in each TIFF file to reduce the number of times I had to enter a file name.

When I got home I used GIMP to extract the individual images into JPEGs. Unexpectedly the JPEGs are enormous! For example one of the TIFF files that was 5MB in total, containing 5 images remember, exported into JPEGs which ranged from 9.0MB to 9.6MB! That is using the default 85% quality.

This is unexpected since TIFF uses lossless compression while I have indicated to GIMP that I would tolerate some quality loss. Even when I reduced the quality to 65% the JPEGs were still over 5MB each.

Are there any suggestions on how I can get GIMP to construct reasonable sized JPEGs?

There may be some obvious answer o your question I don't know about. I hope someone provides it.

But...

It is hard to evaluate your question without knowing some more about the images.

What size was the original tiff file? What were the pixel dimensions of the individual subimages which you wanted to save as jpegs?

What size would they be if you saved them as tiff files?

Among other things, jpeg only compresses significantly if there is sufficient redundancy in the file to do so. If that isn't so, it is conceivable you might not get that much reduction. How about showing us one of those images?

James Cobban wrote:

I am just astonished that I have to set such extreme quality values to get a reasonable sized file.

Very likely the TIFF files are 1 bit per pixel (only black or white, with no gray). JPEG supports a minimum of 8 bits per pixel, so right away, with equivalently efficient compression, the JPEGs are going to be 8 times as large.

JPEG is also most efficient when there aren't a lot of high frequency components (sharp transitions in color or shade), and this is the opposite of what you have with a page full of text.

If you really need these to be JPEG, you can reduce them to, say, half the original size, and still get quite acceptable quality. The lost resolution will be mitigated somewhat by the increased bit depth. This would also make the images a more reasonable size for viewing on a computer monitor.

But if you want to retain as much of the original information as possible, leave them as TIFF or convert to another format that supports losslessly compressed 1-bit images (GIF, for example).

- Ernie http://home.comcast.net/~erniew

On Sat, 30 May 2009 12:50:31 -0400, James Cobban wrote:

In any
event TIFF is able to compress each image to about 1MB using lossless compression, so clearly JPEG should be able to do better.

Not necessarily - the tiff images are probably monochrome, and tiff is exceptionally good at compressing monochrome images. You may get smaller images if you use PNG, but JPEG will almost certainly be bigger, as JPEG is more suitable for full colour images.

Hi gimpusers,

I am just astonished that I have to set such extreme quality values to get a reasonable sized file.

Very likely the TIFF files are 1 bit per pixel (only black or white, with no gray). JPEG supports a minimum of 8 bits per pixel, so right away, with equivalently efficient compression, the JPEGs are going to be 8 times as large.

I think Ernie answered the question. But here's some additional thoughts, that may or may not help solve the issue :-)

Wouldn't Image > Properties provide some additional numerical information to concretely explain file sizes.

Some tidbits:

- Only save an image using a lossy compression once all other image editing has been completed, since many image manipulations can amplify compression artifacts.

- Ensure that image noise levels are as low as possible, since this will produce dramatically smaller JPEG files. http://www.cambridgeincolour.com/tutorials/imagetypes.htm

- JPEG Compression: The real story. http://www.tawbaware.com/maxlyons/jpegcomp.htm

- Color data mode(Bits per pixel) TIF
RGB - 24 or 48 bits,
Grayscale - 8 or 16 bits,
Indexed color - 1 to 8 bits,
Line Art (bilevel)- 1 bit

PNG RGB - 24 or 48 bits,
Grayscale - 8 or 16 bits,
Indexed color - 1 to 8 bits,
Line Art (bilevel) - 1 bit

JPG RGB - 24 bits,
Grayscale - 8 bits

GIF
Indexed color - 1 to 8 bits
http://www.scantips.com/basics09.html

On Sat, 30 May 2009 08:00:05 -0700 (PDT) James Cobban wrote:

When I got home I used GIMP to extract the individual images into JPEGs. Unexpectedly the JPEGs are enormous! For example one of the TIFF files that was 5MB in total, containing 5 images remember, exported into JPEGs which ranged from 9.0MB to 9.6MB! That is using the default 85% quality.

Note that in some cases, lossless compression can give you much better results than jpg. Particularly if the original is a complicated line drawing (high contrast). Also, if the original is using indexed colors, it may be more efficient. And if the original is a b/w 1 bit/pixel image, even more so.

Try to convert to PNG or GIF and check if those formats give you more logical sizes - they work better with the mentioned image types.

John