After compression, the picture's resolution has been reduced based on 1) the
size of the image on the slide and 2) the size of the screen. It all gets a
bit complicated, but think of it this way.
Before compression:
Perhaps your projector uses 1024 pixels width. No matter what, you will not
be able to use more pixels than that.
You insert a picture from your camera that is 3000 pixels wide.
You resize your picture so that it is only 1/2 the slide's width
Remember the screen is only 1024 pixels wide, so if the slide fills the
screen, half of that is 512 pixels.
What happens to the other 2500 (3000 - ~512 = 2500) pixels? Easy, when
windows sends them to the 1024 pixel width projector, it rounds them. It
takes the average of every 36 or so pixels and makes a single average pixel.
Remember this is a 2 dimensional grid, so it isn't just 1:6 horizontally,
but also 1:6 vertically, yielding 1:36 (6x6). Of course it is a bit more
complicated, but that is the concept. The picture is shown at the
resolution available.
It gets more confusing, because windows does not just send the picture to
the projector screen, it also sends it to your monitor screen, which may
have a much higher (or lower) resolution. The user may also have the edit
screen zoomed to just about any size. We'll ignore this part for now.
Still with me? I hope so.
Compression:
When you compress a file, it figures out what size you can use. In our
example above, we can only actually display about 1 pixel of every 36, so
the computer averages the pixel and throws out the un-needed stuff. Because
the image is optimized, the computer has saved all of the usable picture
information, and discarded the detail that can not be used. This has two
benefits and one drawback.
Benefits: smaller files size and faster display. Since we have throw away
35/36ths of the picture's size, then PPT file will be smaller. It will also
be "right-sized" so the computer does not have to do any averaging to figure
out what pixels need to be displayed
Drawback: If you decide later to expand the picture from 1/2 screen to full
screen, it will appear blurry. Your picture's pixel resolution was reduced
to 512, but now it covers 1024 pixels (screen width). It will not look
sharp. That is why I recommended holding onto the originals, if you are
likely to need to resize the images.
If I have completely confused you, don't worry, you are not alone.
I could give you illustrations of looking thru a screen door at stretchy
pictures ... lol
Bill Dilworth
PTtoVideo