在 php 中附加一个 png pHYs 块

Appending a png pHYs chunk in php(在 php 中附加一个 png pHYs 块)

问题描述

我正在尝试添加一些有关物理尺寸的信息，以便在生成 PNG 之前打印它们.

I'm trying to tack on some information about the physical size for printing my PNGs just before they are generated.

阅读 libpng doc 和 pHYs 块规范很有帮助但我似乎无法破解它.

Reading the libpng doc and the pHYs chunk specifications has been helpful but I just can't seem to crack it.

我已尝试以最手动和最简单的方式添加此块，但是 .png 文件最终损坏.我错过了编码技巧吗?

I have tried adding this chunk in the most manual and simplest way possible, however, the .png file ends up corrupted. Am I missing an encoding trick?

对于 CRC 计算，我使用了 this site，已插入下面代码给我的块的 ASCII 值.

For the CRC computation I have used the 32-bit result of this site, having plugged in the ASCII values for the chunk that the code below gives me.

$encoded = $_POST['imgdata'];
$encoded = str_replace(' ', '+', $encoded);
$decoded=  base64_decode($encoded);

$test = explode('IDAT',$decoded);
$ppu='00000000000000000010111000100011';  //32-bit integer for the pixels per unit
$dppu=bindec($ppu);
$test[0].=sprintf("%c",bindec('00000000000000000000000000001001')) //length, also 32-bit
    .'pHYs'                                                      //type
    .  sprintf("%c",$dppu)                             //Pixels per unit, x axis
    . sprintf("%c",$dppu)                             //Pixels per unit, y axis
    .'1'                                                 //Units in metres (1 byte)
    .  sprintf("%c",  bindec(base_convert('0x0BFAAA7E', 16, 2))) //CRC (32-bit)
    .'IDAT';
$fintest=implode($test);

echo $fintest;

请让我知道像这样入侵它是否可行.我也不确定我的 32 位整数:当我正在做正确的方法使它们成为 32 位时，是否对它们进行了零填充?

Please let me know whether hacking it in like this is likely to work. I am also unsure about my 32-bit integers: is zero-padding them as I am doing the correct way to make them 32-bit?

推荐答案

花了一天的时间，我发现这样做的方法是逐字节翻译所有内容.按照文档，pHYs 块需要:

After having spent the day on this, I found that the way to do this was to translate everything byte by byte. Following the docs, the pHYs chunk takes:

• 块(仅数据)长度为 4 个字节
• 4 个字节用于块类型(字符pHYs")
• 9 字节用于块数据:x 和 y 密度各 4 字节，单位选择 1 字节
• 4 个字节用于 CRC

我将所有这些都写成二进制字符串，必要时用 0 填充字节.然后事实证明， chr() 函数正确编码了它以在 PNG 文件中使用，这与我使用的 sprintf("%c",$string) 方法不同在问题中.这是代码

I wrote all this as a binary string, padding the bytes with 0 as necessary. It then turns out that the chr() function correctly encodes this to be used in the PNG file, unlike the sprintf("%c",$string) method I use in the question. Here is the code

$encoded = $_POST['imgdata'];
$encoded = str_replace(' ', '+', $encoded);
$decoded=  base64_decode($encoded);

$binstring='00000000000000000000000000001001' //4-byte length
        . '01110000010010000101100101110011' //4-byte type or 'pHYs'
        . '000000000000000000101110001000110000000000000000001011100010001100000001' //9-byte data
        . base_convert('0x0BFAAA7E', 16, 2); //4-byte CRC
foreach (str_split($binstring,8) as $b) {
    $on.=chr(bindec($b));
}

然后将 $on 变量附加到 IDAT 块长度的 4 个字节之前的位置.

The $on variable is then appended to the position just before the 4 bytes of the IDAT chunk length.

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