Okay .gif geeks please help me

[Dru]

an in paGetop

[forrest_m]

Dru said:

for instance i never get it when photo shop users say saving a quality changes the image size, the two are independent for me...

 

Dru, your question is revealing in its lack of understanding of general principles. The relationship between image quality and file size (regardless of file format) is mathematical and not dependent on your software.

 

The following is grossly oversimplified, but hopefully is helpful. All the formats we're talking about are raster graphics formats, i.e. they’re essentially like a sheet of graph paper with each square called out to be a certain color. Each square of the graph paper is called a "pixel" The factors that determine a file’s size are resolution (i.e. how big are the squares in your graph paper) and pixel depth (how many color choices are there for each square). For example, for black and white images, you only need 1 bit of information per pixel (is it on or off). For gray scale or color you need to save more information per pixel of graphic.

 

.bmp is the dumbest form of graphic information, basically a simple x,y coordinate system with a color designation for each location. .bmp (short for “bit map”) is limited to the system palette of colors.

 

.gif is very similar to .bmp, but the format allows different pixel depths. .gif’s can be very large and high quality or small and low quality, depending on pixel depth.

 

.tif is rather more sophisticated. No display, print, or scanning device can actually create the millions of distinct colors that make up a high quality image. Instead, they use a combination of a couple of simple colors to create these many different colors (red, green and blue for your screen and scanner; cyan, magenta, yellow and black for most color printers). Therefore, if your graphic format actually names the final color for each pixel, each individual computer will have to figure out some way to convert these colors to RGB, and since this is software driven, the same image will print and display differently and, even worse, will slowly morph as it is passed from machine to machine. So what a .tif does is stores the data in the more basic form of channels. A typical .tif has 3 channels, each a grayscale image in which “black” represents 100% of the basic color (red, green, etc). A program like photoshop combines these into a single composite to make them easy to see, but the data remains separated into discrete channels and is thus “uncontaminated” by the idiosyncracies of the software you have chosen to view it in. .tif is not limited to 3 channels; it is typical in the print industry to use 4 channels, one for each color of CMYK ink. The image is scanned in CMYK and printed in CMYK, and color remains constant from initial scan to final print job regardless of the different computers and monitors that are use in the design process. .tif files can also save additional channels that can be used for masking images (i.e. to lay a jacket over a photograph in a catalog) or to allow for various special effects. .tif is generally considered the “best” graphic format because of it’s high fidelity, but its disadvantage is large file size.

 

The .jpg format by definition includes compression technology. When you save as a jpeg, the machine scans the photo for areas of similar color, and then instead of going pixel by pixel, it saves a mathematical description of regions of similar color. In this way, it gains a huge size advantage over .tif files. Two .jpg files that are the same resolution and pixel depth can end up being of different size, because a photo of, say, a crowd of people is less amenable to this kind of compression than, say, a sunset, because there are many more small regions of different color. Jpegs suffer from the same problem as any other compressed format, because it is “lossy.” Small anomalous regions will disappear and the photo will be increasingly “smoothed” over time as it is opened and re-saved. This is apparent, for example, in photos with large areas of color gradation such as skies. Look closely and you will often see that the sky (or ocean or snowfield) becomes “banded” because the jpeg engine has divided what was a smooth transition into discrete bands of color. When the computer asks you to pick a “quality” level when saving the jpeg, what you are doing is deciding how finely the regions are defined. A “highest” or “level 12” jpeg is little different than a .tif because the regions are very small; a level 1 can resemble abstract art. The jpeg format has gained prominence in recent years because the web puts a premium on “good enough” images that transmit quickly.

[klenke]

Hey, Forrest, that's some good stuff. Thanks for the info.

 

This picture kind of shows what Forrest is talking about in terms of banding of the sky. You have to look closely. The banding (gradation) was due to the poster of the picture not uploading a large-size jpeg file. The one he/she uploaded was one they saved at lower jpeg resolution. Makes for quick uploading times but the resultant summitpost picture quality suffers. Note that summitpost only accepts jpeg files.

[Al_Pine]

Yeah , good shit, thanks man.

[scot'teryx]

GIF's are for graphics

JPEG's are for photos

TIFFS are too large for the web, and whomever publishes bmp's on the web are total idiots. my .07

 

Imageready is the best source for creating animated gifs, or just use flash for smaller types of files since they get seriously compacted