Progressive JPEG layer/scan offsets (imagemagick?) - imagemagick

Question

I've been searching high and low for this to no avail. I'd like to be able to extract the byte offset at which each progressive JPEG layer/scan occurs.
For example, let's say a 100 kB image has 5 layers used to render the final image; layer 1 ends at kB 5, layer 2 at kB 20, layer 3 at kB 60, etc (for example).
Is it possible to use Imagemagick for this? The identify tool does not seem to support doing so. If so, how? Otherwise what tool(s) could accomplish this? I'd rather not have to write a custom JPEG parser. Thanks.

Answer 1

You can make a progressive JPEG like this with ImageMagick for testing:
magick -interlace plane -size 400x200 gradient: progressive.jpg
exiftool will tell you quite a lot about it like this:
exiftool -v3 progressive.jpg
Sample Output
ExifToolVersion = 12.00
FileName = progressive.jpg
Directory = .
FileSize = 2709
FileModifyDate = 1620144585
FileAccessDate = 1620144586
FileInodeChangeDate = 1620144585
FilePermissions = 33188
FileType = JPEG
FileTypeExtension = JPG
MIMEType = image/jpeg
JPEG APP0 (14 bytes):
0006: 4a 46 49 46 00 01 01 00 00 01 00 01 00 00 [JFIF..........]
+ [BinaryData directory, 9 bytes]
| JFIFVersion = 1 1
| - Tag 0x0000 (2 bytes, int8u[2]):
| 000b: 01 01 [..]
| ResolutionUnit = 0
| - Tag 0x0002 (1 bytes, int8u[1]):
| 000d: 00 [.]
| XResolution = 1
| - Tag 0x0003 (2 bytes, int16u[1]):
| 000e: 00 01 [..]
| YResolution = 1
| - Tag 0x0005 (2 bytes, int16u[1]):
| 0010: 00 01 [..]
| ThumbnailWidth = 0
| - Tag 0x0007 (1 bytes, int8u[1]):
| 0012: 00 [.]
| ThumbnailHeight = 0
| - Tag 0x0008 (1 bytes, int8u[1]):
| 0013: 00 [.]
JPEG DQT (65 bytes):
0018: 00 03 02 02 02 02 02 03 02 02 02 03 03 03 03 04 [................]
0028: 06 04 04 04 04 04 08 06 06 05 06 09 08 0a 0a 09 [................]
0038: 08 09 09 0a 0c 0f 0c 0a 0b 0e 0b 09 09 0d 11 0d [................]
0048: 0e 0f 10 10 11 10 0a 0c 12 13 12 10 13 0f 10 10 [................]
0058: 10 [.]
JPEG SOF2 (9 bytes):
005d: 08 00 c8 01 90 01 01 11 00 [.........]
ImageWidth = 400
ImageHeight = 200
EncodingProcess = 2
BitsPerSample = 8
ColorComponents = 1
JPEG DHT (20 bytes):
006a: 00 01 01 01 00 00 00 00 00 00 00 00 00 00 00 00 [................]
007a: 00 00 04 08 [....]
JPEG SOS
JPEG DHT (19 bytes):
0139: 10 01 01 00 00 00 00 00 00 00 00 00 00 00 00 00 [................]
0149: 00 00 12 [...]
JPEG SOS
JPEG DHT (18 bytes):
0468: 10 01 00 00 00 00 00 00 00 00 00 00 00 00 00 00 [................]
0478: 00 a0 [..]
JPEG SOS
JPEG DHT (20 bytes):
048a: 10 01 01 01 00 00 00 00 00 00 00 00 00 00 00 00 [................]
049a: 00 71 00 70 [.q.p]
JPEG SOS
JPEG SOS
JPEG DHT (21 bytes):
0728: 10 00 03 01 00 00 00 00 00 00 00 00 00 00 00 00 [................]
0738: 00 01 71 00 50 [..q.P]
JPEG SOS
JPEG EOI
You can also find the SOS markers like this:
xxd -c16 -g1 -u progressive.jpg | grep --color=always -A4 "FF DA"
00000070: 00 00 00 00 00 00 00 00 00 00 00 00 04 08 FF DA ................
00000080: 00 08 01 01 00 00 00 01 D4 60 00 00 00 00 00 08 .........`......
00000090: 40 00 00 00 00 00 11 00 00 00 00 00 00 21 00 00 #............!..
000000a0: 00 00 00 00 42 00 00 00 00 00 00 88 00 00 00 00 ....B...........
000000b0: 00 01 08 00 00 00 00 00 02 10 00 00 00 00 00 04 ................
--
00000140: 00 00 00 00 00 00 00 00 00 00 00 12 FF DA 00 08 ................
00000150: 01 01 00 01 05 02 B5 AD 6B 5A D6 B5 AD 6B 5A D6 ........kZ...kZ.
00000160: B5 AD 6B 5A D6 B5 AD 6B 5A D6 B5 AD 6B 5A D6 B5 ..kZ...kZ...kZ..
00000170: AD 6B 5A D6 B5 AD 6B 5A D6 B5 AD 6B 5A D6 B5 AD .kZ...kZ...kZ...
00000180: 6B 5A D6 B5 AD 6B 5A D6 B5 AD 6B 5A D6 B5 AD 6B kZ...kZ...kZ...k
It may miss some markers if they fall across a 16-byte line end - I am still thinking about making this fool-proof. Maybe run it twice with a 4-byte offset so they can't fall on the boundary in both listings.
Alternatively, I did another answer here that lets you look for arbitrary binary sequences that you could easily adapt from looking for TIFF headers to looking for FF DA.

Answer 2

All you'd have to do is scan the image stream and look for the SOS (Start of Scan) markers.

Answer 3

There is some php example code in this article by Christoph Erdmann:
https://www.smashingmagazine.com/2019/08/faster-image-loading-embedded-previews/#creating-frontend-javascript-code
$img = "progressive.jpg";
$jpgdata = file_get_contents($img);
$positions = [];
$offset = 0;
while ($pos = strpos($jpgdata, "\xFF\xC4", $offset)) {
$positions[] = $pos+2;
$offset = $pos+2;
}