PNG validation on iOS - ios

Question

Writing a mapping application on iOS, making use of OpenStreetMap tiles.
Map tile images are downloaded asynchronously and stored in a dictionary, or persisted in a SQLite DB.
Occasionally, for whatever reason, while attempting to render a map tile image, I get the following error:
ImageIO: <ERROR> PNGinvalid distance too far back
This causes nasty black squares to appear over my map.
This is the piece of code in which this occurs:
NSData *imageData = [TileDownloader RetrieveDataAtTileX:(int)tilex Y:(int)tiley Zoom:(int)zoomLevel];
if (imageData != nil) {
NSLog(#"Obtained image data\n");
UIImage *img = [[UIImage imageWithData:imageData] retain];
// Perform the image render on the current UI context.
// ERROR OCCURS BETWEEN PUSH AND POP
UIGraphicsPushContext(context);
[img drawInRect:[self rectForMapRect:mapRect] blendMode:kCGBlendModeNormal alpha:1.0f];
UIGraphicsPopContext();
[img release];
}
Now, what I'm looking for is a way to ensure a png is valid before attempting to render it to my map.
Edit: The system also occasionally throws this error:
ImageIO: <ERROR> PNGIDAT: CRC error

Answer 1

I found this in other question and put together what solved the issue for me. Hope you find this helpful.
The PNG format has several built in checks. Each "chunk" has a CRC32 check, but to check that you'd need to read the full file.
A more basic check (not foolproof, of course) would be to read the start and ending of the file.
The first 8 bytes should always be the following (decimal) values { 137, 80, 78, 71, 13, 10, 26, 10 } (ref). In particular, the bytes second-to-fourth correspond to the ASCII string "PNG".
In hexadecimal:
89 50 4e 47 0d 0a 1a 0a
.. P N G ...........
You can also check the last 12 bytes of the file (IEND chunk). The middle 4 bytes should correspond to the ASCII string "IEND". More specifically the last 12 bytes should be (in hexa):
00 00 00 00 49 45 4e 44 ae 42 60 82
........... I E N D ...........
(Strictly speaking, it's not really obligatory for a PNG file to end with those 12 bytes, the IEND chunk itself signals the end of the PNG stream and so a file could in principle have extra trailing bytes which would be ignored by the PNG reader. In practice, this is extremely improbable).
Here is an implementation:
- (BOOL)dataIsValidPNG:(NSData *)data
{
if (!data || data.length < 12)
{
return NO;
}
NSInteger totalBytes = data.length;
const char *bytes = (const char *)[data bytes];
return (bytes[0] == (char)0x89 && // PNG
bytes[1] == (char)0x50 &&
bytes[2] == (char)0x4e &&
bytes[3] == (char)0x47 &&
bytes[4] == (char)0x0d &&
bytes[5] == (char)0x0a &&
bytes[6] == (char)0x1a &&
bytes[7] == (char)0x0a &&
bytes[totalBytes - 12] == (char)0x00 && // IEND
bytes[totalBytes - 11] == (char)0x00 &&
bytes[totalBytes - 10] == (char)0x00 &&
bytes[totalBytes - 9] == (char)0x00 &&
bytes[totalBytes - 8] == (char)0x49 &&
bytes[totalBytes - 7] == (char)0x45 &&
bytes[totalBytes - 6] == (char)0x4e &&
bytes[totalBytes - 5] == (char)0x44 &&
bytes[totalBytes - 4] == (char)0xae &&
bytes[totalBytes - 3] == (char)0x42 &&
bytes[totalBytes - 2] == (char)0x60 &&
bytes[totalBytes - 1] == (char)0x82);
}

Answer 2

I know this is a super old thread, but I was looking around for an NSData extension that actually validated the crc32's in the PNG data chunks. Having not found one, I adapted one from some other source.
This will actually flag bad PNG CRC's, which isn't done (shockingly) by most image libraries
static const unsigned int datacrc32_table[256] =
{
0x00000000, 0x77073096, 0xee0e612c, 0x990951ba, 0x076dc419,
0x706af48f, 0xe963a535, 0x9e6495a3, 0x0edb8832, 0x79dcb8a4,
0xe0d5e91e, 0x97d2d988, 0x09b64c2b, 0x7eb17cbd, 0xe7b82d07,
0x90bf1d91, 0x1db71064, 0x6ab020f2, 0xf3b97148, 0x84be41de,
0x1adad47d, 0x6ddde4eb, 0xf4d4b551, 0x83d385c7, 0x136c9856,
0x646ba8c0, 0xfd62f97a, 0x8a65c9ec, 0x14015c4f, 0x63066cd9,
0xfa0f3d63, 0x8d080df5, 0x3b6e20c8, 0x4c69105e, 0xd56041e4,
0xa2677172, 0x3c03e4d1, 0x4b04d447, 0xd20d85fd, 0xa50ab56b,
0x35b5a8fa, 0x42b2986c, 0xdbbbc9d6, 0xacbcf940, 0x32d86ce3,
0x45df5c75, 0xdcd60dcf, 0xabd13d59, 0x26d930ac, 0x51de003a,
0xc8d75180, 0xbfd06116, 0x21b4f4b5, 0x56b3c423, 0xcfba9599,
0xb8bda50f, 0x2802b89e, 0x5f058808, 0xc60cd9b2, 0xb10be924,
0x2f6f7c87, 0x58684c11, 0xc1611dab, 0xb6662d3d, 0x76dc4190,
0x01db7106, 0x98d220bc, 0xefd5102a, 0x71b18589, 0x06b6b51f,
0x9fbfe4a5, 0xe8b8d433, 0x7807c9a2, 0x0f00f934, 0x9609a88e,
0xe10e9818, 0x7f6a0dbb, 0x086d3d2d, 0x91646c97, 0xe6635c01,
0x6b6b51f4, 0x1c6c6162, 0x856530d8, 0xf262004e, 0x6c0695ed,
0x1b01a57b, 0x8208f4c1, 0xf50fc457, 0x65b0d9c6, 0x12b7e950,
0x8bbeb8ea, 0xfcb9887c, 0x62dd1ddf, 0x15da2d49, 0x8cd37cf3,
0xfbd44c65, 0x4db26158, 0x3ab551ce, 0xa3bc0074, 0xd4bb30e2,
0x4adfa541, 0x3dd895d7, 0xa4d1c46d, 0xd3d6f4fb, 0x4369e96a,
0x346ed9fc, 0xad678846, 0xda60b8d0, 0x44042d73, 0x33031de5,
0xaa0a4c5f, 0xdd0d7cc9, 0x5005713c, 0x270241aa, 0xbe0b1010,
0xc90c2086, 0x5768b525, 0x206f85b3, 0xb966d409, 0xce61e49f,
0x5edef90e, 0x29d9c998, 0xb0d09822, 0xc7d7a8b4, 0x59b33d17,
0x2eb40d81, 0xb7bd5c3b, 0xc0ba6cad, 0xedb88320, 0x9abfb3b6,
0x03b6e20c, 0x74b1d29a, 0xead54739, 0x9dd277af, 0x04db2615,
0x73dc1683, 0xe3630b12, 0x94643b84, 0x0d6d6a3e, 0x7a6a5aa8,
0xe40ecf0b, 0x9309ff9d, 0x0a00ae27, 0x7d079eb1, 0xf00f9344,
0x8708a3d2, 0x1e01f268, 0x6906c2fe, 0xf762575d, 0x806567cb,
0x196c3671, 0x6e6b06e7, 0xfed41b76, 0x89d32be0, 0x10da7a5a,
0x67dd4acc, 0xf9b9df6f, 0x8ebeeff9, 0x17b7be43, 0x60b08ed5,
0xd6d6a3e8, 0xa1d1937e, 0x38d8c2c4, 0x4fdff252, 0xd1bb67f1,
0xa6bc5767, 0x3fb506dd, 0x48b2364b, 0xd80d2bda, 0xaf0a1b4c,
0x36034af6, 0x41047a60, 0xdf60efc3, 0xa867df55, 0x316e8eef,
0x4669be79, 0xcb61b38c, 0xbc66831a, 0x256fd2a0, 0x5268e236,
0xcc0c7795, 0xbb0b4703, 0x220216b9, 0x5505262f, 0xc5ba3bbe,
0xb2bd0b28, 0x2bb45a92, 0x5cb36a04, 0xc2d7ffa7, 0xb5d0cf31,
0x2cd99e8b, 0x5bdeae1d, 0x9b64c2b0, 0xec63f226, 0x756aa39c,
0x026d930a, 0x9c0906a9, 0xeb0e363f, 0x72076785, 0x05005713,
0x95bf4a82, 0xe2b87a14, 0x7bb12bae, 0x0cb61b38, 0x92d28e9b,
0xe5d5be0d, 0x7cdcefb7, 0x0bdbdf21, 0x86d3d2d4, 0xf1d4e242,
0x68ddb3f8, 0x1fda836e, 0x81be16cd, 0xf6b9265b, 0x6fb077e1,
0x18b74777, 0x88085ae6, 0xff0f6a70, 0x66063bca, 0x11010b5c,
0x8f659eff, 0xf862ae69, 0x616bffd3, 0x166ccf45, 0xa00ae278,
0xd70dd2ee, 0x4e048354, 0x3903b3c2, 0xa7672661, 0xd06016f7,
0x4969474d, 0x3e6e77db, 0xaed16a4a, 0xd9d65adc, 0x40df0b66,
0x37d83bf0, 0xa9bcae53, 0xdebb9ec5, 0x47b2cf7f, 0x30b5ffe9,
0xbdbdf21c, 0xcabac28a, 0x53b39330, 0x24b4a3a6, 0xbad03605,
0xcdd70693, 0x54de5729, 0x23d967bf, 0xb3667a2e, 0xc4614ab8,
0x5d681b02, 0x2a6f2b94, 0xb40bbe37, 0xc30c8ea1, 0x5a05df1b,
0x2d02ef8d
};
unsigned int
datacrc32 (unsigned int crc, unsigned char *buf, int len)
{
unsigned char *end;
crc = ~crc;
for (end = buf + len; buf < end; ++buf)
crc = datacrc32_table[(crc ^ *buf) & 0xff] ^ (crc >> 8);
return ~crc;
}
-(BOOL)isCRCValidPNG {
char chnk [5];
int l = 0;
int size = (int)[self length];
unsigned int crc = 0;
unsigned char c;
unsigned int csum = 0;
unsigned char b;
unsigned char *tileBytes = (unsigned char *)[self bytes];
if (self.length > 8){
const unsigned char pngHeaderBytes[] = { 0x89, 0x50, 0x4e, 0x47, 0x0d, 0x0a, 0x1a, 0x0a };
for (int i = 0 ; i < 8 ; ++i){
if (tileBytes[i] != pngHeaderBytes[i])
return NO;
}
}
// process chunks
int bytePtr = 8;
strcpy (chnk, "");
do
{
// get chunk size
if (bytePtr+4 > size)
return NO;
l = 0;
for (int i = 0; i < 4; i++)
{
l = (l << 8) + tileBytes[bytePtr++];
}
printf("l is %08x",l);
// get chunk name
crc = 0;
strcpy (chnk, "");
if (bytePtr+4 > size)
return NO;
for (int i = 0; i < 4; i++)
{
c = tileBytes[bytePtr++];
crc = datacrc32 (crc, &c, 1);
chnk[i] = (char) c;
}
chnk[4] = '\0';
printf ("%s (%3d )", chnk, l);
// chunk data
if (bytePtr+l > size)
return NO;
for (int i = 0; i < l; i++)
{
c = tileBytes[bytePtr++];
crc = datacrc32 (crc, &c, 1);
}
// checksum
csum = 0;
if (bytePtr+4 > size)
return NO;
for (int i = 0; i < 4; i++)
{
c = tileBytes[bytePtr++];
csum = (csum << 8) + (int) c;
b = (unsigned char) ((crc >> 8 * (3 - i)) & 0xFF);
// printf ("b = %02x\n", b);
}
if (crc == csum)
NSLog(#"Chunk %s validated",chnk);
else
NSLog(#"chunk %s invalid ",chnk);
if (crc != csum)
return NO;
}
while (strcmp (chnk, "IEND") != 0);
return YES;
}

Answer 3

Switched from my own Asynchronous Download Queue Manager to the All Seeing I implementation. Problem became a moot point.

Answer 4

The Swift Version
func checkPNGImageDataFormat(_ imageData:Data) -> Bool
{
//More expensive since it has to go through entire data
//Check entire header magic number and IEND trailer in PNG data
var status:Bool = true
if(imageData.count < 12)
{
return false
}
let totalBytes = imageData.count
let bytes = imageData.withUnsafeBytes {
[UInt8](UnsafeBufferPointer(start: $0, count: totalBytes))
}
let header:Bool = bytes[0] == 0x89 && bytes[1] == 0x50 && bytes[2] == 0x4e && bytes[3] == 0x47 && bytes[4] == 0x0d && bytes[5] == 0x0a && bytes[6] == 0x1a && bytes[7] == 0x0a
let iend:Bool = bytes[totalBytes - 12] == 0x00 && bytes[totalBytes - 11] == 0x00 && bytes[totalBytes - 10] == 0x00 && bytes[totalBytes - 9] == 0x00 && bytes[totalBytes - 8] == 0x49 && bytes[totalBytes - 7] == 0x45 && bytes[totalBytes - 6] == 0x4e && bytes[totalBytes - 5] == 0x44 && bytes[totalBytes - 4] == 0xae && bytes[totalBytes - 3] == 0x42 && bytes[totalBytes - 2] == 0x60 && bytes[totalBytes - 1] == 0x82
status = header && iend
return status
}