Marco,
Thanks for the exiftool pointer - very nice utility. There is indeed
about 2KB of extra Header info in the Apple JPEG - mostly ICC profile
info. There is also some subsampling differences as the Apple default
is using 4:2:0 and Adobe is using 4:4:4 which may play a factor in
quality.
I'll need to play with these a bit and see if I can find a common
sense method of extracting the unneeded info. It's not a killer but
with thousands of page views per day and web users expecting snappy
page loads I'll throw a little effort into it. For those interested I
here is the header info for the two files as regurgitated from
exiftool - please comment if you see anything glaringly obvious that
should be considered...
//============== ADOBE JPEG ==================//
---- ExifTool ----
ExifTool Version Number : 7.81
---- File ----
File Name : 696-0.8-PS.jpg
Directory : .
File Size : 22 kB
File Modification Date/Time : 2009:06:29 15:36:26-07:00
File Type : JPEG
MIME Type : image/jpeg
Image Width : 305
Image Height : 259
Encoding Process : Baseline DCT, Huffman coding
Bits Per Sample : 8
Color Components : 3
Y Cb Cr Sub Sampling : YCbCr4:4:4 (1 1)
---- JFIF ----
JFIF Version : 1.02
Resolution Unit : None
X Resolution : 100
Y Resolution : 100
---- Ducky ----
Quality : 80%
---- Adobe ----
DCT Encode Version : 100
APP14 Flags 0 : [14], Encoded with Blend=1
downsampling
APP14 Flags 1 : (none)
Color Transform : YCbCr
---- Composite ----
Image Size : 305x259
//============== APPLE JPEG ==================//
---- ExifTool ----
ExifTool Version Number : 7.81
---- File ----
File Name : 696-0.8.jpg
Directory : .
File Size : 28 kB
File Modification Date/Time : 2009:06:30 09:23:05-07:00
File Type : JPEG
MIME Type : image/jpeg
Exif Byte Order : Big-endian (Motorola, MM)
Image Width : 305
Image Height : 259
Encoding Process : Baseline DCT, Huffman coding
Bits Per Sample : 8
Color Components : 3
Y Cb Cr Sub Sampling : YCbCr4:2:0 (2 2)
---- JFIF ----
JFIF Version : 1.01
Resolution Unit : None
X Resolution : 1
Y Resolution : 1
---- ICC-header ----
Profile CMM Type : appl
Profile Version : 2.0.0
Profile Class : Display Device Profile
Color Space Data : RGB
Profile Connection Space : XYZ
Profile Date Time : 2009:05:15 09:32:58
Profile File Signature : acsp
Primary Platform : Apple Computer Inc.
CMM Flags : Not Embedded, Independent
Device Manufacturer :
Device Model :
Device Attributes : Reflective, Glossy, Positive, Color
Rendering Intent : Perceptual
Connection Space Illuminant : 0.9642 1 0.82491
Profile Creator : appl
Profile ID : d3aacba427aa3e36c6c6c0beb9e6dff7
---- ICC_Profile ----
Red Matrix Column : 0.35953 0.21156 0.02415
Green Matrix Column : 0.4537 0.70166 0.12245
Blue Matrix Column : 0.15099 0.08719 0.67818
Media White Point : 0.95047 1 1.0891
Chromatic Adaptation : 1.04788 0.02292 -0.0502 0.02957
0.99049 -0.01706 -0.00923 0.01508 0.75165
Red Tone Reproduction Curve : (Binary data 14 bytes, use -b option
to extract)
Green Tone Reproduction Curve : (Binary data 14 bytes, use -b option
to extract)
Blue Tone Reproduction Curve : (Binary data 14 bytes, use -b option
to extract)
Video Card Gamma : (Binary data 1554 bytes, use -b
option to extract)
Native Display Info : (Binary data 1598 bytes, use -b
option to extract)
Profile Description : Color LCD
Profile Description ML : Color LCD
Make And Model : (Binary data 40 bytes, use -b option
to extract)
Profile Copyright : Copyright Apple, Inc., 2009
---- ExifIFD ----
Exif Image Width : 305
Exif Image Height : 259
---- Composite ----
Image Size : 305x259
On Jun 29, 2009, at 11:24 PM, Marco S Hyman wrote:
On Jun 29, 2009, at 4:12 PM, Chris Tracewell wrote:
The issue is that when using writeToFile my images have an extra 3
to 10 KB of size whether the image is really small or really big -
ie 5KB or 300KB. I am suspecting that my XCode generated images are
bigger for one of two reasons...
1) Adobe's JPEG compression is just plain better and unless I
develop my own JPEG compression algorithm I am hosed.
2) My Objective-C created images contain extra file meta data /
headers that are inflating the size a bit
Possible. Grab a copy of exiftool and run it against a both a
photoshop image and one saved using your method. It will tell
you what metadata is in each image.
I know that very old versions of photoshop would not save the
exif/iptc/whatever metadata which annoyed me no end as I find
the info quite worthwhile to have around. On the other hand,
my "prepare images for web page" script makes a point of stripping
everything out of thumbnail images.
If you can't figure out how to do it in your app take a look
at jhead. Your app could call "jhead -se -purejpg <filename>"
to do the job for you.
// marc
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