Understanding Image Sizes, Before And After Display

[Alejandro Tejada]

Hi All,

In this moment that I could not sleep because the fever,
the mail list is a good reading to keep me awake.

There is another way to read the width and height of
a jpg image. Read the binary data for the Start of Frame
tag within the jpg image.

Please test and improve this script:

on mouseUp
local temp
answer file "Select JPEG image"
put it into temp
open file temp for binary read
read from file temp for 100000
put it into temp1
close file temp

put numtobyte(255) & numtobyte(192) into tBaselineJPGFrame0
-- in Hexadecimal: FFC0 =  JPEG Start-Of-Frame (SOF) marker FFC0
put numtobyte(255) & numtobyte(194) into tProgressiveJPGFrame0
-- in Hexadecimal: FFC2 =  JPEG Start-Of-Frame (SOF) marker FFC2

set the casesensitive to true

put offset(tBaselineJPGFrame0,temp1) into tStartOFrame
if tStartOFrame is 0 then put offset(tProgressiveJPGFrame0,temp1) into
tStartOFrame

if tStartOFrame is not empty
then
put byte (tStartOFrame + 5) of temp1 & byte (tStartOFrame + 6) of temp1
into tJPGHeight
put byte (tStartOFrame + 7) of temp1 & byte (tStartOFrame + 8) of temp1
into tJPGWidth

-- put baseConvert(baseConvert(bytetonum(byte 1 of tJPGHeight),10,16) &
baseConvert(bytetonum(byte 2 of tJPGHeight),10,16),16,10) into tJPGHeight

put bytetonum(byte 1 of tJPGHeight) into q
put bytetonum(byte 2 of tJPGHeight) into w
put baseConvert(q,10,16) into r
if the number of bytes of r = 1 then put "0" & r into r
put baseConvert(w,10,16) into t
if the number of bytes of t = 1 then put "0" & t into t
put r & t into y
put baseConvert(y,16,10) into tJPGHeight

-- put baseConvert(baseConvert(bytetonum(byte 1 of tJPGWidth),10,16) &
baseConvert(bytetonum(byte 2 of tJPGWidth),10,16),16,10) into tJPGWidth

put bytetonum(byte 1 of tJPGWidth) into q
put bytetonum(byte 2 of tJPGWidth) into w
put baseConvert(q,10,16) into r
if the number of bytes of r = 1 then put "0" & r into r
put baseConvert(w,10,16) into t
if the number of bytes of t = 1 then put "0" & t into t
put r & t into y
put baseConvert(y,16,10) into tJPGWidth

answer "JPG Width" && tJPGWidth & cr & "JPG Height" && tJPGHeight & cr &
temp

else
answer "Could not find Start of Frame information in this JPEG image" & cr
& temp
end if

end mouseUp

 /*
Information from
https://stackoverflow.com/questions/35427283/get-width-and-height-from-jpeg-without-0xff-0xc0

The JPEG Start-Of-Frame (SOF) marker has 4 possible values:

    FFC0 (baseline) - This is the usual mode chosen for photos and encodes
fully specified DCT blocks in groupings depending on the color/subsample
options chosen
    FFC1 (extended) - This is similar to baseline, but has more than 8-bits
per color stimulus
    FFC2 (progressive) - This mode is often found on web pages to allow the
image to load progressively as the data is received. Each "scan" of the
image progressively defines more coefficients of the DCT blocks until
they're fully defined. This effectively provides more and more detail as
more scans are decoded
    FFC3 (lossless) - This mode uses a simple Huffman encoding to
losslessly encode the image. The only place I've seen this used is on
16-bit grayscale DICOM medical images

 */