Point Sampling
In the case where an image is scaled to 1/4th its original size (also
referred to as down-sampling), the new pixel in
the destination will represents an array of 4x4 pixels in the source
image. In the nearest neighbor algorithm, only one pixel is chosen
to represent all of its neighbors. This causes a loss of detail and
missing pixel errors
in the scaled image (see point sampling image above).
Interpolated Sampling
Good scaling algorithms use interpolation to create the destination
pixels. In the 4:1 down-sampling example discussed above, all the pixels
in the 4x4 area are taken into account in calculating the destination
pixel. By allowing all the pixels in the source image to contribute to the
destination image, the picture will not lose the pixel data (see
Interpolated sampling image above). Interpolated images sometime appear
blurry and are therefore sharpened a little bit.
GIF Scaling
A GIF image is an indexed-color image, meaning that the colors of the
image have been reduced to small set of colors that are used over and over
in the image. To properly scale a GIF image, it must first be converted to
an RGB image so that pixels can be interpolated. This is accomplished by
converting the image to RGB using the "Image->Mode" menu in a photo editor
like Adobe Photoshop or Photoshop Elements (Photoshop Elements is a lower
cost version of Photoshop that can do the same functions for less money).
Convert image to RGB mode
Once the image is converted to RGB, it can
then be resampled or scaled down to a smaller size. In the illustration
below, the original pixel size was 200 and it was then set to 100. Once
the "OK" button is pressed, the image now appears smaller.
Scale Image using Bicubic or Bilinear Resampling
When down sampling an image, it often
appears blurry. Using the Filter->Sharpen->Unsharp Mask... menu in
Photoshop, you can sharpen the image. The typical settings for web images
is shown below:
Sharpen Image
Saving the Scaled GIF Image
Once the image has been sharpened, it is time to save it as a .gif file
again. In Photoshop, this is accomplished via the "File->Save for Web..."
menu.
Save for Web
As stated above, the GIF image is an
index-color image meaning that the colors of the image have been reduced
to a limited set of colors, usually 4, 8, 16, 32, 64 or 128 colors. The
reduction of colors is one of the reasons GIF images are more compact then
other image types. The "Save for Web" dialog allow you to view the result
of the color reduction process when saving a GIF image.
Save as a GIF Image
In the "Save for Web" dialog you can adjust
the gif image depth, look at the new image size and make sure the preview
is adequate. Once the image colors are optimum, press the "Save" button to
save the image. Then import the image into your FrontPage web for use.
This same process can be used up-sampling
of images or for JPEG images that "fall apart" in the down-sampling
process.
