就是JPEG最小截取编码块 8x8的大小。
引述地址：
https://www.impulseadventure.com/photo/rotation-partial-mcu.html
https://www.impulseadventure.com/photo/jpeg-minimum-coded-unit.html
https://quippe.eu/blog/2016/11/17/determining-minimum-coded-unit-dimensions.html
源码：get_mcu_dimensions.sh
file=$1 # Get position and length of SOF0 header in file. sof0hex=$(exiv2 -p S $file | grep SOF0 | sed 's/|.*|//') IFS=' ' read -a sof0hexfields <<< "${sof0hex}" offset=${sof0hexfields[0]} length=${sof0hexfields[1]} # Read SOF0 values into array. sof0string=$(hexdump "$1" -s $offset -n $length -v -e '/1 "x "' | \ sed 's/\ $//') read -a sof0 <<< "${sof0string}" # Check if length of SOF0 is as expected. sof0length=${#sof0[@]} sof0explength=$((16#${sof0[0]}${sof0[1]})) if [ ${#sof0[@]} -ne $((16#${sof0[0]}${sof0[1]})) ]; then echo "Length of SOF0 in bytes ($sof0length) not as expected ($sof0explength)." exit fi # Check if the image is a YCbCr image (the only encoding this script handles). if [ ${sof0[7]} -ne 3 ]; then echo "Image has ${sof0[7]} instead of 3 components: not YCbCr." exit fi if [ ${sof0[14]} -ne 3 ]; then echo "Image is not YCbCr (most likely YIQ instead, or else just screwed)." exit fi # Check if Cb and Cr are both 11. if [ ${sof0[12]} -ne 11 -o ${sof0[15]} -ne 11 ]; then echo "Sampling factors of Cb and/or Cr is/are not equal to 11." exit fi # Determine MCU based on Y component sampling factors: y_hor=$(echo ${sof0[9]} | cut -c 1) y_ver=$(echo ${sof0[9]} | cut -c 2) mcu_x=$((y_hor * 8)) mcu_y=$((y_ver * 8)) echo -e $"mcu_x\t$mcu_x" echo -e $"mcu_y\t$mcu_y"
使用如下：
ubuntu@ubuntu:~$ ./get_mcu_dimensions.sh example_16x16.jpg mcu_x 16 mcu_y 16
