What is compress actually doing?

[John]

Problem: What is compress actually doing?



Word 2007

XP Pro SP2



If you compress the images in a DOC file using Format Pictures... | Picture tab | Compress button, the following happens:

The file becomes smaller.

The printed page looks the same.

The screen image looks the same.



What actually changed?

Is it the Resolution in DPI?

Is it the Dimension in Pixels?



Changing the Resolution in DPI should only change the physical size of how something prints. It does not change the number of pixels in the image itself.



Changing the Dimension in Pixels would cut off part of the image, or

it could do an algorithmic shrink where there are less Dimension in Pixels so you see the same image but using fewer Dimension in Pixels. I appreciate your feedback.

 

[Terry Farrell]

Compress changes the resolution. If you click on Compress, Options, you can
choose between Print 220ppi, Screen 96 ppi or email 72 ppi. (This is for
Word 2007: other versions are slightly different.)

--
Terry Farrell - MSWord MVP

Problem: What is compress actually doing?

Word 2007
XP Pro SP2

If you compress the images in a DOC file using Format Pictures... | Picture
tab | Compress button, the following happens:
The file becomes smaller.
The printed page looks the same.
The screen image looks the same.

What actually changed?
Is it the Resolution in DPI?
Is it the Dimension in Pixels?

Changing the Resolution in DPI should only change the physical size of how
something prints. It does not change the number of pixels in the image
itself.

Changing the Dimension in Pixels would cut off part of the image, or
it could do an algorithmic shrink where there are less Dimension in Pixels
so you see the same image but using fewer Dimension in Pixels. I appreciate
your feedback.

 

[BoniM]

It's reducing the ppi to only as high as it needs to be for the desired task.
Usually, the reason there is such a big change is not just because of the
change in ppi, but because it also resizes to the actual size in use in the
document. When you insert a huge picture into Word, it will automatically be
resized to fit between the margins and many users resize even smaller.
Click compress pictures and then the options button to control the level of
compression and choose whether you want cropped areas permanently removed or
not.

 

[Klaus Linke]

There are many possible ways to compress a picture file, and different
graphics formats and applications use different ones.
Reducing the resolution is one of them (one that looses information),
reducing the color depth (number of allowed colors) could be another.
If the bitmap was uncompressed before, it can be compressed without any loss
by storing it more efficiently.
Say in a black-and-white picture, you don't need to store the information
that a whole line of say 1000 pixels is white by storing the number for
"white" 1000 times (1000 numbers), you can instead store the number for
white and that there's a run of 1000 such pixels (2 numbers).
There are clever ways to do that (both lossy and lossless) like the ones
used in JPEGs, which basically don't store the pixel colors themselves, but
the parameters for an algorithm to recreate them.

Klaus

 

[Suzanne S. Barnhill]

But I think what the OP meant was "What is Word doing when you use its
Compress Pictures command?"

 

[John]

Thank you for the feedback.



I suspect WORD is using a lossy algorithmic shrink to change the Dimension
of Pixels to say 320x240. What is confusing is that the dialog says Print
200 dpi or Screen 96 dpi which does not change the Dimension of Pixels but
changes how many pixels of the Dimension of Pixels are printed per inch.
However, the physical size of the printed object and screen display do not
change after the compression. Therefore, by specifying 200 dpi (lower
density), WORD would need to make the Dimension of Pixels smaller in order
to have the same physical size.



It would be beneficial for Microsoft to clarify what they are doing.

 

[Klaus Linke]

I think you misunderstand what "dpi" means.

You wrote:
"Changing the Resolution in DPI should only change the physical size of how
something prints. It does not change the number of pixels in the image
itself."

It's just the other way around. Changing the resolution of the picture
should not change its physical size. It does change the number of pixels in
the image.

Many graphics formats specify the size of the image inside the graphics
file. It is quite independent of the resolution (dpi).

If you go down with the resolution (say from an image meant for printing
with 600 dpi to one for the web with 96 dpi), you can do with a lot less
pixels, and/or with a lot more agressive (lossy) compression.

Klaus

 

[John]

Klaus, you are right. Thanks



There is an interesting discussion in Wikipedia under "Dots per inch".

http://en.wikipedia.org/wiki/Dots_per_inch



Resolution = (PhysicalSize) x (DPI)

where

Resolution is in dots i.e. related to physical size of the image file.

PhysicalSize is length of picture on screen or printer.

DPI is dots per length.



When you compress, WORD assumes the PhysicalSize is constant. It specifies a 200 DPI value for a printer. Therefore, it reduces the Resolution i.e. compresses the size of the image file to whatever makes the equation work.



In contrast, when you scale the picture using the handles, the Physical Size changes while the Resolution remains constant. Therefore, the DPI changes accordingly.