Message-Id: <mailto:199503110014.SAA16010@library.wustl.edu> Date: Fri, 10 Mar 1995 17:10:31 -0500 From: Sandeep Somaiya <mailto:sandeep@NEXT2.VTLS.COM> Subject: Re: JPEG and image size To: Multiple recipients of list IMAGELIB
kim brookes <mailto:k_brookes@harvard.edu> wrote: >> 1. What degree, or percentage, or whatever the proper term is, have people
>> used without losing an unacceptable amount of image information,
>>2. can someone give me an example of the change in file size using this
>> degree.
JPEG lets one select file size vs. image quality tradeoff by selecting a quality setting. This quality setting or Q-factor usually ranges from 0-100 or 0-4 or a high-medium-low scale or excellent-good-normal-bad kind of a scale depending on yor software. This scale is basically arbitary and not really a % scale from what I have read. We use the 0-100 scale. Now to address the question of what should be the quality setting or Q-factor or the smallest file size without losing an unacceptable amount of image information .... it is dependent on the following factors: a. It varies from image to image b. and "unacceptable" varies from user to user etc. But a general rule of thumb according to JPEG frequently asked questions. Assume a scale of 0-100. - good quality full color images: set_quality_setting ÷5; (if objectionable_loss) set_quality_setting > 75 till acceptable image quality obtained; below 75 you will see visible loss/defects. - if less than perfect quality image set_quality_setting õ0; (if objectionable_loss) set_quality_setting > 50; [>50 also necessary if image contains dithering or moire patterns] - donot use ( quality_setting > 95 ) - (file_size_of_100_quality_setting) ò to 3 times (file_size_of_95_quality_setting) but hardly any quality difference - if image for preview and indexing only use quality factor > 5 and < 10 24-bit color photographs: JPEG is lossy. Lossless-JPEG 2:1 JPEG will give you 10:1 to 20:1 without visible loss JPEG will give you 30:1 to 50:1 with moderate loss JPEG will give you 100:1 with significant loss Gray-scale 256 shades 8-bit images: JPEG will give visible loss after about 5:1
These are approximations. Results vary from image-to-image and acceptable quality defination varies from observer-to-observer. And standard disclaimer applies.
Best wishes. Sandeep Somaiya VTLS Inc. "Look, Learn, Listen .. Libraries change Lives"