Rasters and world files and extents, oh my!

[David Badke]

I am confused...

I have a raster image, 10540 pixels wide by 6740 pixels high, non-Geo 
TIFF. The specified units in the map file are meters. I created a world 
file:

1
0.0000000000
0.0000000000
-1
0.00
0.00

meaning x co-ordinates increase moving right, y coordinates decrease 
moving down, from top-left pixel at x=0, y=0, with 1 pixel/meter (ie: 
the raster image is 10.54 km wide by 6.74 km high, in world coordinates).

Through trial and error, with many blank output images, I found this 
extent setting (in the map file) produces a correct image, generated 
through a simple PHP MapScript:

EXTENT 0 -6739 10539 0

PHP is:

if (!extension_loaded('MapScript'))
    dl('php_mapscript_46.'.PHP_SHLIB_SUFFIX);
$oMap = ms_newMapObj( '/ms4w/apps/kamap/htdocs/map/vicbird1878.map' );
$oImg = $oMap->draw();
$szURL = $oImg->saveWebImage();
echo "<img src=$szURL>";

While this produces the image I expect, the y coordinate is always 
negative for any x, y point, which is a problem for me - and anyway, I 
don't understand why miny has to be -6739. So I tried a world file like:

1
0.0000000000
0.0000000000
1
0.00
0.00

and an extent like:

EXTENT 0 0 10539 6739

thinking that both x and y coordinates would be positive and increase 
from x=0, y=0 (upper left) to x=10539, y=6739 (bottom right), which is 
what I want. Doesn't work - the image is blank.

So can someone explain in simple terms for the 
cartographically-challenged just what the relationship is between the 
world file and the extent minx, miny, maxx, maxy? If minx, miny is the 
upper left pixel, and the world file says that the upper left pixel is 
x=0, y=0, why doesn't EXTENT 0 0 10539 6739 refer to the entire image? 
Why does a miny value of -6739 in the extent work if the top left is 0,0 
and the y pixel size is -1? Sooooo confused...

David