JPEG compression and quality level

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Karsten Wutzke - 31 May 2005 15:24 GMT

Hello all!

Is there a way or some (free) 3rd party algorithm to determine the JPEG
compression level? We need to make sure that some JPEGs have a certain
quality. Images under a some threshold value shall be rejected form
being upploaded.

E.g. our JPEG quality scale would be from 0-100%, then all images below
70% can't be submitted. (100% is the lowest compression possible, large
file)

Is there such an algorithm?

Karsten

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Harald - 02 Jun 2005 21:40 GMT

> Is there a way or some (free) 3rd party algorithm to determine the
> JPEG compression level? We need to make sure that some JPEGs have a
> certain quality. Images under a some threshold value shall be rejected
> form being upploaded.

If the compression program does not list it in a header entry, I doubt
this is possible without some really sophisticated image analysis. The
ratio

image size*number of pixels/size in bytes

may be a very rough indication if you are expecting photos of known
content. For a general image --- in particular artificially prepared
images from some graphics program --- whatever you see after
decompression may be exactly what was intended. In the end its all
just pixels.

Harald.

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Lucy - 03 Jun 2005 06:33 GMT

> > Is there a way or some (free) 3rd party algorithm to determine the
> > JPEG compression level? We need to make sure that some JPEGs have a
[quoted text clipped - 12 lines]
> decompression may be exactly what was intended. In the end its all
> just pixels.

Sometimes there is info about it in the jpg file if the camera put it there.
Here is the info that I extracted from a jpg file using ExifReader.
Here the compression seems to be 2 to 1 in some sense.

Filename : 103-0330_IMG.JPG
JFIF_APP1 : Exif
Main Information
Make : Canon
Model : Canon PowerShot S40
DateTime : 2005:03:22 19:30:56
Sub Information
ExposureTime : 1/1000Sec
FNumber : F6.3
ExifVersion : 0220
DateTimeOriginal : 2005:03:22 19:30:56
DateTimeDigitized : 2005:03:22 19:30:56
CompressedBitsPerPixel : 2/1 (bit/pixel)
ShutterSpeedValue : 1/1002Sec
ApertureValue : F6.3
ExposureBiasValue : EV0.0
MaxApertureValue : F2.8
MeteringMode : Division
Flash : Not fired(Compulsory)
FocalLength : 8.63(mm)
UserComment :
ExifImageWidth : 2272
ExifImageHeight : 1704
ExposureMode : Auto
WhiteBalance : Auto
DigitalZoomRatio : 2272/2272
SceneCaptureType : Landscape
Vendor Original Information
MacroMode : Off
Self-timer : Off
Quality : Normal
FlashMode : Off
Drive Mode : Single-frame
Focus Mode : Single
ImageSize : Large
Easy shooting mode : Landscape
Digital Zoom : Off
Contrast : Normal
Saturation : Normal
Sharpness : Normal
CCD Sensitivity : AUTO
MeteringMode : Evaluative
FocusType : Auto
AF point selected : auto-selected
ExposureProgram : Easy shooting
Focal length of lens : 7.09375-21.3125(mm)
Flash Activity : Off
Long Shutter Mode : Off
Photo Effect : Neutral
White Balance : Auto
Sequence number(Continuous mode) : 0
Focused AF Area(AiAF) : Left Center Right
Flash bias : 0 EV
Image type : IMG:PowerShotS40JPEG
Firmware version : FirmwareVersion1.10
Image Number : 1030330
Owner name :

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JPEG compression and quality level

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