Introduction

Original Article: http://www.ampsoft.net/webdesign-l/jpeg-compression.html
Article by: Alberto Martinez Perez

The image compression techniques have their origin in the necessity of reducing the size of the file used to store the image, which is huge when compared to storing text (a 800×600 24 bit color uncompressed image needs 1440000 bytes or 1.37 MB). One of the most popular formats is JPEG (which uses files with JPG or JPEG extensions), which can achieve compression ratios of 10:1 with almost no perceived loss of the image quality.

Depending of the program that you use for saving JPEG images, there are available some options for adjusting the compression of the image. In this article I’ll try to explain how to adjust that options and optimize the ratio between image quality and file size. It’s important take into account that some images are more suitable than others for JPEG compression (see flat images compression below).
Basic compression options

The two basic options that are almost always present in every “Save JPEG” dialog are the compression/quality of the image, and the standard/optimized/progressive formats.

Both compression and quality refers to the same parameter but one is the inverse of the other (e.g. in a percentage-based scale 35% compression is equal to 65% quality). Depending of your program of choice you’ll find either, but the result is the same: the more compression, the lesser image quality. The proper setting for this parameter depends of the image quality or file size you need/want to obtain, getting the best ratios with 50%-85% of image quality (equivalent to 50%-15% compression).

The standard (baseline) and optimized (baseline optimized) settings provides the same image quality but the optimized format provides a 2%-8% reduction of the image file size, so there is no reason for not use always this setting when not using the progressive format. The progressive format also provides the same quality, but offers an alternate method of displaying the image while it is downloaded, at the cost of a slight increase in the file size (more details about progressive encoding).
Advanced options

Apart from that options, some programs like Corel PhotoPaint, Paint Shop Pro and IrfanView (this one is freeware) allow choosing the type of image subsampling, also called downsampling. The subsampling ratios according to the spec are 4:4:4 (no subsampling), 4:2:2 (standard subsampling) and 4:1:1 (worst quality, rarely used). Adobe Photoshop also contemplates this option but it’s adjusted automatically depending on the quality level selected: it applies subsampling for Medium and Low levels and uses no subsampling for Maximum and High (more details about subsampling/downsampling).

While the standard subsampling is usually adequate for most images, providing an good ratio between image quality and file size, there are some situations in which using no subsampling (4:4:4) provides a noticeable increase in the image quality, even if you use a higher compression ratio in order to maintain the file size. The most notable cases are when the image contains some portions with fine details, like text over an uniform background, and images that contain almost-flat colors.

Further optimizations

Almost all digital cameras store by default EXIF data in the pictures taken with them. This data include information about the camera configuration and conditions at the time of the photo shot. This information usually takes a lot of space, so make sure that saving this data is disabled in your graphical application when optimizing JPEG files (e.g.: in a test, an 800×600 JPEG picture saved at 75% quality was reduced from 114 KB to 51 KB after stripping the EXIF information).

IrfanView, a freeware graphic viewer/converter, has a plugin for JPEG lossless transformations that allows stripping this data (and other similar like IPTC information) from a JPEG file without recompressing the image itself. In this way you can remove the unnecessary information while maintaining the original image quality.
Flat images compression

Since the JPEG format was designed for photos, images that are composed mainly from solid colors achieve a better compression with formats like PNG and GIF (this one only supports up to 256 colors per image), and without any loss of quality (lossless compression):
Solid colors – standard JPEG subsampling

JPEG image, 15% compression (85% quality), standard subsampling (4:2:2), 2193 bytes.
Solid colors – no JPEG subsampling

JPEG image, 35% compression (65% quality), no subsampling (4:4:4), 2224 bytes.
Solid colors – PNG compression

PNG Image, 24-bit color, best compression (level 9), 1108 bytes.
Solid colors – GIF compression

GIF image, 4-bit color (16 color palette), non-interlaced, 916 bytes.

But if we use the lossless image formats for the first samples, the non-flat portion of the images makes the final file size much bigger:
Mixed JPEG image with no subsampling

JPEG image, 25% compression (75% quality), no subsampling (4:4:4), 8096 bytes.
Mixed image with PNG compression

PNG Image, 24-bit color, best compression (level 9), 24649 bytes.
Mixed image with GIF compression

GIF image, 8-bit color (256 color palette), non-interlaced, 17783 bytes.
Almost flat JPEG image with no subsampling

JPEG image, 25% compression (75% quality), no subsampling (4:4:4), 5151 bytes.
Almost flat image with PNG compression

PNG Image, 24-bit color, best compression (level 9), 24287 bytes.
Almost flat image with GIF compression

GIF image, 8-bit color (256 color palette), non-interlaced, 13031 bytes.
Recompression

Since the JPEG use lossy compression, saving the same JPEG image over and over will cause degradation in the image quality even if you use the same quality settings, specially if you have enabled subsampling. Due to this is recommended maintaining a “master” copy saved in a lossless format (PNG, TIFF or proprietary formats like PSD or CPT) in case you need to make some editing to the image or adjust the JPEG compression settings (a JPEG image previously saved at 40% compression will not improve quality for saving it again at 15% compression).

If for some reason you prefer (or need) to use JPEG for your master copies, use 0% compression (100% quality) with no subsampling, since this reduce the loss of quality near to zero.

See Original Article: http://www.ampsoft.net/webdesign-l/jpeg-compression.html
Article by: Alberto Martinez Perez
amp@ampsoft.net

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