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I'd like to screen some jpegs for validity before I send them across the network for more extensive inspection. It is easy enough to check for a valid header and footer, but what is the smallest size (in bytes) a valid jpeg could be?
390
Although this is slightly smaller than the 125 byte arithmetic encoding given in another answer, the arithmetic encoding without the JFIF header would be smaller yet (107 bytes), so that should still be considered the smallest known.
Finally, the JPEG image (. jpg) is the smallest image and is a full 24 bit color image but it is compressed using a lossy compression scheme.
JPEG/JFIF supports a maximum image size of 65,535×65,535 pixels, hence up to 4 gigapixels for an aspect ratio of 1:1. In 2000, the JPEG group introduced a format intended to be a successor, JPEG 2000, but it was unable to replace the original JPEG as the dominant image standard.
A 1x1 grey pixel in 125 bytes using arithmetic coding, still in the JPEG standard even if most decoders can't decode it:
ff d8 : SOI ff e0 ; APP0 00 10 4a 46 49 46 00 01 01 01 00 48 00 48 00 00 ff db ; DQT 00 43 00 03 02 02 02 02 02 03 02 02 02 03 03 03 03 04 06 04 04 04 04 04 08 06 06 05 06 09 08 0a 0a 09 08 09 09 0a 0c 0f 0c 0a 0b 0e 0b 09 09 0d 11 0d 0e 0f 10 10 11 10 0a 0c 12 13 12 10 13 0f 10 10 10 ff c9 ; SOF 00 0b 08 00 01 00 01 01 01 11 00 ff cc ; DAC 00 06 00 10 10 05 ff da ; SOS 00 08 01 01 00 00 3f 00 d2 cf 20 ff d9 ; EOI I don't think the mentioned 134 byte example is standard, as it is missing an EOI. All decoders will handle this but the standard says it should end with one.
That file can be generated with:
#!/usr/bin/env bash printf '\xff\xd8' # SOI printf '\xff\xe0' # APP0 printf '\x00\x10' printf '\x4a\x46\x49\x46\x00\x01\x01\x01\x00\x48\x00\x48\x00\x00' printf '\xff\xdb' # DQT printf '\x00\x43' printf '\x00' printf '\x03\x02\x02\x02\x02\x02\x03\x02' printf '\x02\x02\x03\x03\x03\x03\x04\x06' printf '\x04\x04\x04\x04\x04\x08\x06\x06' printf '\x05\x06\x09\x08\x0a\x0a\x09\x08' printf '\x09\x09\x0a\x0c\x0f\x0c\x0a\x0b' printf '\x0e\x0b\x09\x09\x0d\x11\x0d\x0e' printf '\x0f\x10\x10\x11\x10\x0a\x0c\x12' printf '\x13\x12\x10\x13\x0f\x10\x10\x10' printf '\xff\xc9' # SOF printf '\x00\x0b' printf '\x08\x00\x01\x00\x01\x01\x01\x11\x00' printf '\xff\xcc' # DAC printf '\x00\x06\x00\x10\x10\x05' printf '\xff\xda' # SOS printf '\x00\x08' printf '\x01\x01\x00\x00\x3f\x00\xd2\xcf\x20' printf '\xff\xd9' # EOI and opened fine with GNOME Image Viewer 3.38.0 and GIMP 2.10.18 on Ubuntu 20.10.
Here's an upload on Imgur. Note that Imgur process the file making it larger however if you download it to check, and as seen below, the width=100 image shows white on Chromium 87:
138
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