PNGMetadataExtractor.php

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<?php
class PNGMetadataExtractor {
    private static $pngSig;
 
    private static $crcSize;
 
    private static $textChunks;
 
    public const VERSION = 1;
    private const MAX_CHUNK_SIZE = 3145728; // 3 mebibytes
 
    public static function getMetadata( $filename ) {
        self::$pngSig = pack( "C8", 137, 80, 78, 71, 13, 10, 26, 10 );
        self::$crcSize = 4;
        /* based on list at http://owl.phy.queensu.ca/~phil/exiftool/TagNames/PNG.html#TextualData
         * and https://www.w3.org/TR/PNG/#11keywords
         */
        self::$textChunks = [
            'xml:com.adobe.xmp' => 'xmp',
            # Artist is unofficial. Author is the recommended
            # keyword in the PNG spec. However some people output
            # Artist so support both.
            'artist' => 'Artist',
            'model' => 'Model',
            'make' => 'Make',
            'author' => 'Artist',
            'comment' => 'PNGFileComment',
            'description' => 'ImageDescription',
            'title' => 'ObjectName',
            'copyright' => 'Copyright',
            # Source as in original device used to make image
            # not as in who gave you the image
            'source' => 'Model',
            'software' => 'Software',
            'disclaimer' => 'Disclaimer',
            'warning' => 'ContentWarning',
            'url' => 'Identifier', # Not sure if this is best mapping. Maybe WebStatement.
            'label' => 'Label',
            'creation time' => 'DateTimeDigitized',
            /* Other potentially useful things - Document */
        ];
 
        $frameCount = 0;
        $loopCount = 1;
        $text = [];
        $duration = 0.0;
        $width = 0;
        $height = 0;
        $bitDepth = 0;
        $colorType = 'unknown';
 
        if ( !$filename ) {
            throw new Exception( __METHOD__ . ": No file name specified" );
        } elseif ( !file_exists( $filename ) || is_dir( $filename ) ) {
            throw new Exception( __METHOD__ . ": File $filename does not exist" );
        }
 
        $fh = fopen( $filename, 'rb' );
 
        if ( !$fh ) {
            throw new Exception( __METHOD__ . ": Unable to open file $filename" );
        }
 
        // Check for the PNG header
        $buf = self::read( $fh, 8 );
        if ( $buf != self::$pngSig ) {
            throw new Exception( __METHOD__ . ": Not a valid PNG file; header: $buf" );
        }
 
        // Read chunks
        while ( !feof( $fh ) ) {
            $buf = self::read( $fh, 4 );
            $chunk_size = unpack( "N", $buf )[1];
 
            if ( $chunk_size < 0 || $chunk_size > self::MAX_CHUNK_SIZE ) {
                wfDebug( __METHOD__ . ': Chunk size of ' . $chunk_size .
                    ' too big, skipping. Max size is: ' . self::MAX_CHUNK_SIZE );
                if ( fseek( $fh, 4 + $chunk_size + self::$crcSize, SEEK_CUR ) !== 0 ) {
                    throw new Exception( __METHOD__ . ': seek error' );
                }
                continue;
            }
 
            $chunk_type = self::read( $fh, 4 );
            $buf = self::read( $fh, $chunk_size );
            $crc = self::read( $fh, self::$crcSize );
            $computed = crc32( $chunk_type . $buf );
            if ( pack( 'N', $computed ) !== $crc ) {
                wfDebug( __METHOD__ . ': chunk has invalid CRC, skipping' );
                continue;
            }
 
            if ( $chunk_type == "IHDR" ) {
                $width = unpack( 'N', substr( $buf, 0, 4 ) )[1];
                $height = unpack( 'N', substr( $buf, 4, 4 ) )[1];
                $bitDepth = ord( substr( $buf, 8, 1 ) );
                // Detect the color type in British English as per the spec
                // https://www.w3.org/TR/PNG/#11IHDR
                switch ( ord( substr( $buf, 9, 1 ) ) ) {
                    case 0:
                        $colorType = 'greyscale';
                        break;
                    case 2:
                        $colorType = 'truecolour';
                        break;
                    case 3:
                        $colorType = 'index-coloured';
                        break;
                    case 4:
                        $colorType = 'greyscale-alpha';
                        break;
                    case 6:
                        $colorType = 'truecolour-alpha';
                        break;
                    default:
                        $colorType = 'unknown';
                        break;
                }
            } elseif ( $chunk_type == "acTL" ) {
                if ( $chunk_size < 4 ) {
                    wfDebug( __METHOD__ . ": acTL chunk too small" );
                    continue;
                }
 
                $actl = unpack( "Nframes/Nplays", $buf );
                $frameCount = $actl['frames'];
                $loopCount = $actl['plays'];
            } elseif ( $chunk_type == "fcTL" ) {
                $buf = substr( $buf, 20 );
                if ( strlen( $buf ) < 4 ) {
                    wfDebug( __METHOD__ . ": fcTL chunk too small" );
                    continue;
                }
 
                $fctldur = unpack( "ndelay_num/ndelay_den", $buf );
                if ( $fctldur['delay_den'] == 0 ) {
                    $fctldur['delay_den'] = 100;
                }
                if ( $fctldur['delay_num'] ) {
                    $duration += $fctldur['delay_num'] / $fctldur['delay_den'];
                }
            } elseif ( $chunk_type == "iTXt" ) {
                // Extracts iTXt chunks, uncompressing if necessary.
                $items = [];
                if ( preg_match(
                    '/^([^\x00]{1,79})\x00(\x00|\x01)\x00([^\x00]*)(.)[^\x00]*\x00(.*)$/Ds',
                    $buf, $items )
                ) {
                    /* $items[1] = text chunk name, $items[2] = compressed flag,
                     * $items[3] = lang code (or ""), $items[4]= compression type.
                     * $items[5] = content
                     */
 
                    // Theoretically should be case-sensitive, but in practise...
                    $items[1] = strtolower( $items[1] );
                    if ( !isset( self::$textChunks[$items[1]] ) ) {
                        // Only extract textual chunks on our list.
                        continue;
                    }
 
                    $items[3] = strtolower( $items[3] );
                    if ( $items[3] == '' ) {
                        // if no lang specified use x-default like in xmp.
                        $items[3] = 'x-default';
                    }
 
                    // if compressed
                    if ( $items[2] == "\x01" ) {
                        if ( function_exists( 'gzuncompress' ) && $items[4] === "\x00" ) {
                            Wikimedia\suppressWarnings();
                            $items[5] = gzuncompress( $items[5] );
                            Wikimedia\restoreWarnings();
 
                            if ( $items[5] === false ) {
                                // decompression failed
                                wfDebug( __METHOD__ . ' Error decompressing iTxt chunk - ' . $items[1] );
                                continue;
                            }
                        } else {
                            wfDebug( __METHOD__ . ' Skipping compressed png iTXt chunk due to lack of zlib,'
                                . " or potentially invalid compression method" );
                            continue;
                        }
                    }
                    $finalKeyword = self::$textChunks[$items[1]];
                    $text[$finalKeyword][$items[3]] = $items[5];
                    $text[$finalKeyword]['_type'] = 'lang';
                } else {
                    // Error reading iTXt chunk
                    wfDebug( __METHOD__ . ": Invalid iTXt chunk" );
                }
            } elseif ( $chunk_type == 'tEXt' ) {
                // In case there is no \x00 which will make explode fail.
                if ( strpos( $buf, "\x00" ) === false ) {
                    wfDebug( __METHOD__ . ": Invalid tEXt chunk: no null byte" );
                    continue;
                }
 
                list( $keyword, $content ) = explode( "\x00", $buf, 2 );
                if ( $keyword === '' ) {
                    wfDebug( __METHOD__ . ": Empty tEXt keyword" );
                    continue;
                }
 
                // Theoretically should be case-sensitive, but in practise...
                $keyword = strtolower( $keyword );
                if ( !isset( self::$textChunks[$keyword] ) ) {
                    // Don't recognize chunk, so skip.
                    continue;
                }
                Wikimedia\suppressWarnings();
                $content = iconv( 'ISO-8859-1', 'UTF-8', $content );
                Wikimedia\restoreWarnings();
 
                if ( $content === false ) {
                    wfDebug( __METHOD__ . ": Read error (error with iconv)" );
                    continue;
                }
 
                $finalKeyword = self::$textChunks[$keyword];
                $text[$finalKeyword]['x-default'] = $content;
                $text[$finalKeyword]['_type'] = 'lang';
            } elseif ( $chunk_type == 'zTXt' ) {
                if ( function_exists( 'gzuncompress' ) ) {
                    // In case there is no \x00 which will make explode fail.
                    if ( strpos( $buf, "\x00" ) === false ) {
                        wfDebug( __METHOD__ . ": No null byte in zTXt chunk" );
                        continue;
                    }
 
                    list( $keyword, $postKeyword ) = explode( "\x00", $buf, 2 );
                    if ( $keyword === '' || $postKeyword === '' ) {
                        wfDebug( __METHOD__ . ": Empty zTXt chunk" );
                        continue;
                    }
                    // Theoretically should be case-sensitive, but in practise...
                    $keyword = strtolower( $keyword );
 
                    if ( !isset( self::$textChunks[$keyword] ) ) {
                        // Don't recognize chunk, so skip.
                        continue;
                    }
                    $compression = substr( $postKeyword, 0, 1 );
                    $content = substr( $postKeyword, 1 );
                    if ( $compression !== "\x00" ) {
                        wfDebug( __METHOD__ . " Unrecognized compression method in zTXt ($keyword). Skipping." );
                        continue;
                    }
 
                    Wikimedia\suppressWarnings();
                    $content = gzuncompress( $content );
                    Wikimedia\restoreWarnings();
 
                    if ( $content === false ) {
                        // decompression failed
                        wfDebug( __METHOD__ . ' Error decompressing zTXt chunk - ' . $keyword );
                        continue;
                    }
 
                    Wikimedia\suppressWarnings();
                    $content = iconv( 'ISO-8859-1', 'UTF-8', $content );
                    Wikimedia\restoreWarnings();
 
                    if ( $content === false ) {
                        wfDebug( __METHOD__ . ": iconv error in zTXt chunk" );
                        continue;
                    }
 
                    $finalKeyword = self::$textChunks[$keyword];
                    $text[$finalKeyword]['x-default'] = $content;
                    $text[$finalKeyword]['_type'] = 'lang';
                } else {
                    wfDebug( __METHOD__ . " Cannot decompress zTXt chunk due to lack of zlib. Skipping." );
                }
            } elseif ( $chunk_type == 'tIME' ) {
                // last mod timestamp.
                if ( $chunk_size !== 7 ) {
                    wfDebug( __METHOD__ . ": tIME wrong size" );
                    continue;
                }
 
                // Note: spec says this should be UTC.
                $t = unpack( "ny/Cm/Cd/Ch/Cmin/Cs", $buf );
                $strTime = sprintf( "%04d%02d%02d%02d%02d%02d",
                    $t['y'], $t['m'], $t['d'], $t['h'],
                    $t['min'], $t['s'] );
 
                $exifTime = wfTimestamp( TS_EXIF, $strTime );
 
                if ( $exifTime ) {
                    $text['DateTime'] = $exifTime;
                }
            } elseif ( $chunk_type == 'pHYs' ) {
                // how big pixels are (dots per meter).
                if ( $chunk_size !== 9 ) {
                    wfDebug( __METHOD__ . ": pHYs wrong size" );
                    continue;
                }
 
                $dim = unpack( "Nwidth/Nheight/Cunit", $buf );
                if ( $dim['unit'] == 1 ) {
                    // Need to check for negative because php
                    // doesn't deal with super-large unsigned 32-bit ints well
                    if ( $dim['width'] > 0 && $dim['height'] > 0 ) {
                        // unit is meters
                        // (as opposed to 0 = undefined )
                        $text['XResolution'] = $dim['width']
                            . '/100';
                        $text['YResolution'] = $dim['height']
                            . '/100';
                        $text['ResolutionUnit'] = 3;
                        // 3 = dots per cm (from Exif).
                    }
                }
            } elseif ( $chunk_type == "IEND" ) {
                break;
            }
        }
        fclose( $fh );
 
        if ( $loopCount > 1 ) {
            $duration *= $loopCount;
        }
 
        if ( isset( $text['DateTimeDigitized'] ) ) {
            // Convert date format from rfc2822 to exif.
            foreach ( $text['DateTimeDigitized'] as $name => &$value ) {
                if ( $name === '_type' ) {
                    continue;
                }
 
                // @todo FIXME: Currently timezones are ignored.
                // possibly should be wfTimestamp's
                // responsibility. (at least for numeric TZ)
                $formatted = wfTimestamp( TS_EXIF, $value );
                if ( $formatted ) {
                    // Only change if we could convert the
                    // date.
                    // The png standard says it should be
                    // in rfc2822 format, but not required.
                    // In general for the exif stuff we
                    // prettify the date if we can, but we
                    // display as-is if we cannot or if
                    // it is invalid.
                    // So do the same here.
 
                    $value = $formatted;
                }
            }
        }
 
        return [
            'width' => $width,
            'height' => $height,
            'frameCount' => $frameCount,
            'loopCount' => $loopCount,
            'duration' => $duration,
            'text' => $text,
            'bitDepth' => $bitDepth,
            'colorType' => $colorType,
        ];
    }
 
    private static function read( $fh, $size ) {
        if ( $size === 0 ) {
            return '';
        }
 
        $result = fread( $fh, $size );
        if ( $result === false ) {
            throw new Exception( __METHOD__ . ': read error' );
        }
        if ( strlen( $result ) < $size ) {
            throw new Exception( __METHOD__ . ': unexpected end of file' );
        }
        return $result;
    }
}