我有很多大小不同的jpeg文件。例如，这里是一个大小为256 * 384（像素）的图像的hexdump给出的前640个字节：
0000000: ffd8 ffe0 0010 4a46 4946 0001 0101 0048 ......JFIF.....H
0000010: 0048 0000 ffdb 0043 0003 0202 0302 0203 .H.....C........
0000020: 0303 0304 0303 0405 0805 0504 0405 0a07 ................
0000030: 0706 080c 0a0c 0c0b 0a0b 0b0d 0e12 100d ................
我猜测大小信息必须在这些行中。但是我无法看到哪些字节正确地提供了大小。有谁能帮助我找到包含大小信息的字段吗？
这里有一部分C代码可能对你有所帮助：解码JPEG（JFIF）文件的宽度和高度
这个函数将读取JPEG属性。
function jpegProps(data) { // data is an array of bytes
var off = 0;
while(off<data.length) {
while(data[off]==0xff) off++;
var mrkr = data[off]; off++;
if(mrkr==0xd8) continue; // SOI
if(mrkr==0xd9) break; // EOI
if(0xd0<=mrkr && mrkr<=0xd7) continue;
if(mrkr==0x01) continue; // TEM
var len = (data[off]<<8) | data[off+1]; off+=2;
if(mrkr==0xc0) return {
bpc : data[off], // precission (bits per channel)
h : (data[off+1]<<8) | data[off+2],
w : (data[off+3]<<8) | data[off+4],
cps : data[off+5] // number of color components
}
off+=len-2;
}
}
我已经将顶部答案中的 CPP 代码转换为 Python 脚本。
"""
Source: https://dev59.com/y3E85IYBdhLWcg3w_Itr:~:text=The%20header%20of%20a%20JPEG,Of%20Frame%2C%20type%20N).
"""
def get_jpeg_size(data):
"""
Gets the JPEG size from the array of data passed to the function, file reference: http:#www.obrador.com/essentialjpeg/headerinfo.htm
"""
data_size=len(data)
#Check for valid JPEG image
i=0 # Keeps track of the position within the file
if(data[i] == 0xFF and data[i+1] == 0xD8 and data[i+2] == 0xFF and data[i+3] == 0xE0):
# Check for valid JPEG header (null terminated JFIF)
i += 4
if(data[i+2] == ord('J') and data[i+3] == ord('F') and data[i+4] == ord('I') and data[i+5] == ord('F') and data[i+6] == 0x00):
#Retrieve the block length of the first block since the first block will not contain the size of file
block_length = data[i] * 256 + data[i+1]
while (i<data_size):
i+=block_length #Increase the file index to get to the next block
if(i >= data_size): return False; #Check to protect against segmentation faults
if(data[i] != 0xFF): return False; #Check that we are truly at the start of another block
if(data[i+1] == 0xC0): #0xFFC0 is the "Start of frame" marker which contains the file size
#The structure of the 0xFFC0 block is quite simple [0xFFC0][ushort length][uchar precision][ushort x][ushort y]
height = data[i+5]*256 + data[i+6];
width = data[i+7]*256 + data[i+8];
return height, width
else:
i+=2; #Skip the block marker
block_length = data[i] * 256 + data[i+1] #Go to the next block
return False #If this point is reached then no size was found
else:
return False #Not a valid JFIF string
else:
return False #Not a valid SOI header
with open('path/to/file.jpg','rb') as handle:
data = handle.read()
h, w = get_jpeg_size(data)
print(s)
以下是我使用js实现的方法。您要查找的标记是Sofn标记，伪代码基本上是：
FF开头，后跟另一个字节表示标记类型（这两个字节称为标记）01或D1到D9，则该段中没有数据，请继续下一段C0或C2（或任何其他Cn，有关详细信息，请参见代码注释），那么就是您要查找的Sofn标记
function getJpgSize(hexArr) {
let i = 0;
let marker = '';
while (i < hexArr.length) {
//ff always start a marker,
//something's really wrong if the first btye isn't ff
if (hexArr[i] !== 'ff') {
console.log(i);
throw new Error('aaaaaaa');
}
//get the second byte of the marker, which indicates the marker type
marker = hexArr[++i];
//these are segments that don't have any data stored in it, thus only 2 bytes
//01 and D1 through D9
if (marker === '01' || (!isNaN(parseInt(marker[1])) && marker[0] === 'd')) {
i++;
continue;
}
/*
sofn marker: https://www.w3.org/Graphics/JPEG/itu-t81.pdf pg 36
INFORMATION TECHNOLOGY –
DIGITAL COMPRESSION AND CODING
OF CONTINUOUS-TONE STILL IMAGES –
REQUIREMENTS AND GUIDELINES
basically, sofn (start of frame, type n) segment contains information
about the characteristics of the jpg
the marker is followed by:
- Lf [frame header length], two bytes
- P [sample precision], one byte
- Y [number of lines in the src img], two bytes, which is essentially the height
- X [number of samples per line], two bytes, which is essentially the width
... [other parameters]
sofn marker codes: https://www.digicamsoft.com/itu/itu-t81-36.html
apparently there are other sofn markers but these two the most common ones
*/
if (marker === 'c0' || marker === 'c2') {
break;
}
//2 bytes specifying length of the segment (length excludes marker)
//jumps to the next seg
i += parseInt(hexArr.slice(i + 1, i + 3).join(''), 16) + 1;
}
const size = {
height: parseInt(hexArr.slice(i + 4, i + 6).join(''), 16),
width: parseInt(hexArr.slice(i + 6, i + 8).join(''), 16),
};
return size;
}
从论坛中的一个解决方案移植到Dart/Flutter。
class JpegProps {
final int precision;
final int height;
final int width;
final int compression;
JpegProps._(this.precision, this.height, this.width, this.compression,);
String toString() => 'JpegProps($precision,$height,$width,$compression)';
static JpegProps readImage(Uint8List imageData) {
// data is an array of bytes
int offset = 0;
while (offset < imageData.length) {
while (imageData[offset] == 0xff) offset++;
var mrkr = imageData[offset];
offset++;
if (mrkr == 0xd8) continue; // SOI
if (mrkr == 0xd9) break; // EOI
if (0xd0 <= mrkr && mrkr <= 0xd7) continue;
if (mrkr == 0x01) continue; // TEM
var length = (imageData[offset] << 8) | imageData[offset + 1];
offset += 2;
if (mrkr == 0xc0) {
return JpegProps._(imageData[offset],
(imageData[offset + 1] << 8) | imageData[offset + 2],
(imageData[offset + 3] << 8) | imageData[offset + 4],
imageData[offset + 5],
);
}
offset += length - 2;
}
throw '';
}
}
从 .jpg 图片中获取宽度和高度的简单方法。删除文件中的 EXIF 和 ITP 信息。使用一个查看图片程序的 "另存为" 功能（我使用的是 IrfanView 或 Pain Shop Pro）。在 "另存为" 中去掉 EXIF，然后保存文件。jpg 文件没有 EXIF 时，高度位于字节 000000a3 和 000000a4，宽度位于 000000a5 和 000000a6。
我使用 PHP。
function storrelse_jpg($billedfil) //billedfil danish for picturefile
{
//Adresse for jpg fil without EXIF info !!!!!
// width is in byte 165 til 166, heigh is in byte 163 og 164
// jpg dimensions are with 2 bytes ( in png are the dimensions with 4 bytes
$billedfil="../diashow/billeder/christiansdal_teltplads_1_x.jpg"; // the picturefil
$tekst=file_get_contents($billedfil,0,NULL,165,2); //Read from 165 2 bytes - width
$tekst1=file_get_contents($billedfil,0,NULL,163,2);//Read from 163 2 bytes - heigh
$n=strlen($tekst); // længden af strengen
echo "Størrelse på billed : ".$billedfil. "<br>"; // Headline
$bredde=0; // width
$langde=0; // heigh
for ($i=0;$i<$n;$i++)
{
$by=bin2hex($tekst[$i]); //width-byte from binær to hex
$bz=hexdec($by);// then from hex to decimal
$ly=bin2hex($tekst1[$i]); // the same for length byte
$lz=hexdec($ly);
$bredde=$bredde+$bz*256**(1-$i);
$langde=$langde+$lz*256**(1-$i);
}
// $x is a array $x[0] er width and $x[1] er heigh
$x[0]=$bredde; $x[1]=$langde;
return $x;
}
def get_jpeg_resolution(image_bytes: bytes,
size: int = None) -> Optional[Tuple[int, int]]:
"""
function for getting resolution from binary
:param image_bytes: image binary
:param size: image_bytes len if value is None it'll calc inside
:return: (width, height) or None if not found
"""
size = len(image_bytes) if size is None else size
header_bytes = (0xff, 0xD8, 0xff, 0xe0)
if not (size > 11
and header_bytes == struct.unpack_from('>4B', image_bytes)):
# Incorrect header or minimal length
return None
jfif_bytes = tuple(ord(s) for s in 'JFIF') + (0x0, )
if not (jfif_bytes == struct.unpack_from('5B', image_bytes, 6)):
# Not a valid JFIF string
return None
index = len(header_bytes)
block_length, = struct.unpack_from(">H", image_bytes, index)
index += block_length
while index < size:
if image_bytes[index] != 0xFF:
break
# Check that we are truly at the start
# of another block
if image_bytes[index + 1] == 0xC0:
# 0xFFC0 is the "Start of frame" marker
# which contains the file size
# The structure of the 0xFFC0 block is
# quite simple
# [0xFFC0][ushort length][uchar precision]
# [ushort x][ushort y]
height, width = struct.unpack_from(">HH", image_bytes, index + 5)
return width, height
else:
index += 2
# Skip the block marker
# Go to the next block
block_length, = struct.unpack(">H",
image_bytes[slice(index, index + 2)])
# Increase the file index to get to the next block
index += block_length
# If this point is reached then no size was found
return None
#! /usr/bin/php -q
<?php
if (file_exists($argv[1]) ) {
$targetfile = $argv[1];
// get info on uploaded file residing in the /var/tmp directory:
$safefile = escapeshellcmd($targetfile);
$getinfo = `/usr/bin/identify $safefile`;
$imginfo = preg_split("/\s+/",$getinfo);
$ftype = strtolower($imginfo[1]);
$fsize = $imginfo[2];
switch($fsize) {
case 0:
print "FAILED\n";
break;
default:
print $safefile.'|'.$ftype.'|'.$fsize."|\n";
}
}
// eof
主机> imageinfo 009140_DJI_0007.JPG
009140_DJI_0007.JPG|jpeg|4000x3000|
（以管道分隔的格式输出文件名、文件类型和文件尺寸）
来自 man 手册：
要了解有关“identify”命令的更多信息，请将您的浏览器指向[...] http://www.imagemagick.org/script/identify.php。