PHP测试图像是否隔行扫描

Question

I have a folder that will consist of hundreds of PNG files and I want to write a script to make them all interlaced. Now, images will be added to that folder over time and processing all the images in the folder (wether their interlaced or progressive) seems kinda silly.

So I was wondering, is there any way to use PHP to detect if an image is interlaced or not that way I can choose wether to process it or not.

Thanks heaps!

Answer 1

You can also take the low-level approach - no need of loading the full image, or to use extra tools or libraries. If we look at the spec, we see that the "interlaced" flag it's just the byte 13 of the iHDR chunk, so we have to skip 8 bytes from the signature, plus 8 bytes of the iHDR Chunk identifier+length, plus 12 bytes of the chunk... That gives 28 bytes to be skipped, and if the next byte is 0 then the image is not interlaced.

The implementation takes just 4 lines of code:

function isInterlaced( $filename ) {
   $handle = fopen($filename, "r");
   $contents = fread($handle, 32);
   fclose($handle);
   return( ord($contents[28]) != 0 );
}

BTW, are you sure you want to use interlaced PNG? (see eg)