Get size of image without loading in to memory

Question

I have several .png images (ETA: but the format could also be JPEG or something else) that I am going to display in UITableViewCells. Right now, in order to get the row heights, I load in the images, get their size properties, and use that to figure out how high to make the rows (calculating any necessary changes along the way, since most of the images get resized before being displayed). In order to speed things up and reduce memory usage, I'd like to be able to get size without loading the images. Is there a way to do this?

Note: I know that there are a number of shortcuts I could implement to eliminate this issue, but for several reasons I can't resize images in advance or collect the image sizes in advance, forcing me to get this info at run time.

Answer 1

Note: This function doesn't work with iPhone compressed PNGs, this compression is automatically performed by XCode and change the image header, see more details here and how to disable this feature: http://discussions.apple.com/thread.jspa?threadID=1751896

Future versions of PSFramework will interpret this headers too, stay tuned.

---

See this function, she does just that. Reads only 30 bytes of the PNG file and returns the size (CGSize). This function is part of a framework for processing images called PSFramework (http://sourceforge.net/projects/photoshopframew/). Not yet implemented for other image formats, developers are welcome. The project is Open Source under the GNU License.

CGSize PSPNGSizeFromMetaData( NSString* anFileName ) {

    // File Name from Bundle Path.
    NSString *fullFileName = [NSString stringWithFormat:@"%@/%@", [[NSBundle mainBundle] bundlePath], anFileName ];

    // File Name to C String.
    const char* fileName = [fullFileName UTF8String];

    /* source file */ 
    FILE * infile;    

    // Check if can open the file.
    if ((infile = fopen(fileName, "rb")) == NULL) 
    {
        NSLog(@"PSFramework Warning >> (PSPNGSizeFromMetaData) can't open the file: %@", anFileName );
        return CGSizeZero;

    }

    //////  //////  //////  //////  //////  //////  //////  //////  //////  //////  ////// 

    // Lenght of Buffer.
    #define bytesLenght 30

    // Bytes Buffer.
    unsigned char buffer[bytesLenght];

    // Grab Only First Bytes.
    fread(buffer, 1, bytesLenght, infile);

    // Close File.
    fclose(infile);

    //////  //////  //////  //////  ////// 

    // PNG Signature.
    unsigned char png_signature[8] = {137, 80, 78, 71, 13, 10, 26, 10};

    // Compare File signature.
    if ((int)(memcmp(&buffer[0], &png_signature[0], 8))) {

        NSLog(@"PSFramework Warning >> (PSPNGSizeFromMetaData) : The file (%@) don't is one PNG file.", anFileName);    
        return CGSizeZero;

    }

    //////  //////  //////  //////  ////// //////   //////  //////  //////  ////// 

    // Calc Sizes. Isolate only four bytes of each size (width, height).
    int width[4];
    int height[4];
    for ( int d = 16; d < ( 16 + 4 ); d++ ) {
        width[ d-16] = buffer[ d ];
        height[d-16] = buffer[ d + 4];
    }

    // Convert bytes to Long (Integer)
    long resultWidth = (width[0] << (int)24) | (width[1] << (int)16) | (width[2] << (int)8) | width[3]; 
    long resultHeight = (height[0] << (int)24) | (height[1] << (int)16) | (height[2] << (int)8) | height[3]; 

    // Return Size.
    return CGSizeMake( resultWidth, resultHeight );

}

Answer 2

low tech solutions:

if you know what the images are beforehand, store the image sizes along with their filenames in an XML file or plist (or whichever way you prefer) and just read those properties in.

if you don't know what the images are (i.e. they're going to be defined at runtime), then you must've had the images loaded at one time or another. the first time you do have them loaded, save their height and width in a file so you can access it later.