Image file type comparison (JPEG & GIF)

The software I used for manipulating these pictures is GIMP version-2.8.6

The original images I worked with are PNG files.  I will explore the difference in quality and size when exporting these images as JPEG’s.

JPEG files are a widely used format that compresses images in an effective way while minimising the loss of image quality.  When saving a file in JPEG format there is an option to set the quality level, the lower the quality the smaller the file size you will produce. 

I will also explore the relationship between the size and number of colours in GIF images, both with and without dithering.  The maximum number of colours supported by GIF images is 256 and reducing this number will reduce the size of the file.  When the colours are reduced it may become noticeable to see banding across similar colours, I will use dithering to compromise for this and the image will appear to have higher quality.

JPEG Files

       Figure 1.1 (original PNG file at 484KB)

I will use this image to compare differences in JPEG quality.

The first file I exported from this image was at 100 percent quality as a JPEG image which reduced the file size to 394KB with no visible loss of quality.

The first sign of loss of quality was when I reduced the level of quality below 80 percent but this was minimal loss.  Noticeable difference was at 40 percent quality, with the file size of less than 35.7KB

       Fig1.2 (the same image at 40 percent quality in JPEG format at 35.7KB)

You may notice the loss of quality in details like the designs and patterns on the couch and armchair.
The 484KB original is more than 13 times the size of the reduced quality file at 35.7KB

  

Smoothing

Some of the sharpness is lost with reduced quality, but there is an option to increase the smoothness of the image which will have the effect of blurring the image and reducing the size of the image. 

     Fig 1.3 (Comparison of JPEG at 40 percent from original)

        Image with .4 smoothing (28.2KB)                                               Image with no smoothing (35.7KB)

The difference in perceivable quality is small but for this image it looks better with no smoothing because it looks sharper.

GIF Files

       Figure 2.1 (original PNG file at 390KB)

I will use this image to compare differences in GIF files.

To change the number of colours in an image you need to change the mode of the image from RGB to indexed colour selection.   To so this is simple - Select image, mode and click on indexed, this will give you the options for number of colours and options for dithering.

This picture was converted at maximum size and quality with 256 colours with no visible loss of quality but a large reduction on file size (127KB).

The first sign of loss of quality was when I reduced the number of colours to 64.  The banding is noticeable in the sky as shown in figure 2.2. 

       Figure 2.2 (image 2 with 64 colours in gif format at 71.6KB)

At this point I will attempt to camouflage the banding by using dithering.  

The figure below (Figure 2.3) had 64 colours using dithering.

       Figure 2.3(image 2 with 64 colours and dithering in gif format at 101KB)

This file is larger than the same image with the same number of colours because I used dithering.  The image looks much better because the effect of banding is removed.

     Close up section of Figure 2.2                                   Close up section of Figure 2.3

I have found that reducing the quality of JPEG images to anything below 40 percent will take too much from the image.  The size of the files will be tremendously reduced.  This makes it more efficient when downloading the image to view on a webpage.

The GIF images can reduce the number of colours to a small number, but this is very dependent on the picture.  Images with a wide range of shades and colours would need to have a higher number of colours than images with a small amount of shades and colours.