posted on Jun, 23 2007 @ 09:20 AM
Originally posted by harry20007
You may used that facility abit but you also altered the exposure!
How does one 'alter the exposure' after the photograph is taken. You clearly understand very little about digital image manipulation.
Now that photograph that is posted only has the contrast modified. There is no brightness modification present.
Let me explain to you how digital image manipulation works.
Images on the computer are made up of millions of little squares called 'pixels'. Each pixel typically is assigned 3 bytes of data. These three
bytes are the three channels of Red Green and Blue. Each channel has an intensity ranging from 0 - 255.
When I change the brightness of an image in a digital image manipulation software program the program simply goes through each one of these pixels and
adjusts the RGB values based on the fair intensity calculation (that is the calculation which turns colour to black and white) to give a uniform
lowering in overall brightness. This has absolutely nothing to do with exposure, which cannot be changed after the photograph has been taken, unless
the film is exposed accidentally, or rerun somehow.
When I modify the contrast of an image in a digital image manipulation software program, the program will reduce the number of colours allowed to be
on screen and round any colours that do not fit into this set down. That is to say, the more I increase contrast the more each blue will begin to look
the same shade of blue. Same for white, green and red. It is setting a definite palette for the image and forcing the pixels into the palette, so the
image is now made up of colours which are mathematical round downs of their originals.
So when I want to find slight variations in colour I can do the following: turn up the contrast to make the number of possible colours in the image
very few, then change the brightness setting, which has the effect of moving the colour margins back and forth so different regions of colour fall
into different margins. This is because the contrast is calculated from the original image, after the brightness is calculated.
The process is perhaps a tad hard to understand in its internal workings, but it is very intuitive.
The main point I want to make here is what is done to one pixel, is done to all others. The images produced are still uniformly accurate
representations of the pixels from the original image, just with a filter or modifier on them. There has been no 'drawing onto' the image to
inconsistently change some areas and not others.