iPhone Objective C: UIImage - Region of Interest

Question

Is there any way to get the min rectangle area which contains all the non-transparent part of an UIImage?

Reading pixel by pixel to check where alpha == 0 ...isn't a good way I believe.

Any better way?

Many thanks for reading

Answer 1

I don't think there's a way to do this without examining the image pixel by pixel. Where are the images coming from? If you control them, you can at least do the pixel by pixel part only once and then either cache the information or distribute it along with the images if people are downloading them.

Answer 2

Okay here is my ugly solution to this problem. I hope there is a better way to do this.

- (CGRect) getROIRect:(UIImage*)pImage {

CGRect roiRect = {{0,0}, {0,0}};
int vMaxX = -999;
int vMinX = 999;
int vMaxY = -999;
int vMinY = 999;
int x,y;

CGImageRef inImage = pImage.CGImage;
// Create off screen bitmap context to draw the image into. Format ARGB is 4 bytes for each pixel: Alpa, Red, Green, Blue
CGContextRef cgctx = [self createARGBBitmapContextFromImage:inImage];
if (cgctx == NULL) { return roiRect; /* error */ }

size_t w = CGImageGetWidth(inImage);
size_t h = CGImageGetHeight(inImage);
CGRect rect = {{0,0},{w,h}}; 

// Draw the image to the bitmap context. Once we draw, the memory
// allocated for the context for rendering will then contain the
// raw image data in the specified color space.
CGContextDrawImage(cgctx, rect, inImage); 

// Now we can get a pointer to the image data associated with the bitmap
// context.
unsigned char* data ;
BOOL tSet = NO;

    data= CGBitmapContextGetData (cgctx);
if (data != NULL) {
 for (x=0;x<w;x++) {
  for (y=0;y<h;y++) {

   //offset locates the pixel in the data from x,y.
   //4 for 4 bytes of data per pixel, w is width of one row of data.
   int offset = 4*((w*round(y))+round(x));
   int alpha =  data[offset];
   if (alpha > 0) {
    tSet = YES;
    if (x > vMaxX) {
     vMaxX = x;
    }
    if (x < vMinX) {
     vMinX = x;
    }
    if (y > vMaxY) {
     vMaxY = y;
    }
    if (y < vMinY) {
     vMinY = y;
    }
   }
  }
 }
}

if (!tSet) {
 vMaxX = w;
 vMinX = 0;
 vMaxY = h;
 vMinY = 0;
}
// When finished, release the context
CGContextRelease(cgctx);
// Free image data memory for the context
if (data) { free(data); }

CGRect roiRect2 = {{vMinX,vMinY},{vMaxX - vMinX,vMaxY - vMinY}}; 

return roiRect2;
}

- (CGContextRef) createARGBBitmapContextFromImage:(CGImageRef) inImage {

CGContextRef    context = NULL;
CGColorSpaceRef colorSpace;
void *          bitmapData;
int             bitmapByteCount;
int             bitmapBytesPerRow;

// Get image width, height. We'll use the entire image.
size_t pixelsWide = CGImageGetWidth(inImage);
size_t pixelsHigh = CGImageGetHeight(inImage);

// Declare the number of bytes per row. Each pixel in the bitmap in this
// example is represented by 4 bytes; 8 bits each of red, green, blue, and
// alpha.
bitmapBytesPerRow   = (pixelsWide * 4);
bitmapByteCount     = (bitmapBytesPerRow * pixelsHigh);

// Use the generic RGB color space.
//colorSpace = CGColorSpaceCreateWithName(kCGColorSpaceGenericRGB);

colorSpace = CGColorSpaceCreateDeviceRGB();

if (colorSpace == NULL)
{
 fprintf(stderr, "Error allocating color space\n");
 return NULL;
}

// Allocate memory for image data. This is the destination in memory
// where any drawing to the bitmap context will be rendered.
bitmapData = malloc( bitmapByteCount );
if (bitmapData == NULL)
{
 fprintf (stderr, "Memory not allocated!");
 CGColorSpaceRelease( colorSpace );
 return NULL;
}

// Create the bitmap context. We want pre-multiplied ARGB, 8-bits
// per component. Regardless of what the source image format is
// (CMYK, Grayscale, and so on) it will be converted over to the format
// specified here by CGBitmapContextCreate.
context = CGBitmapContextCreate (bitmapData, pixelsWide, pixelsHigh, 8,    bitmapBytesPerRow, colorSpace,  kCGImageAlphaPremultipliedFirst);
if (context == NULL)
{
 free (bitmapData);
 fprintf (stderr, "Context not created!");
}

// Make sure and release colorspace before returning
CGColorSpaceRelease( colorSpace );

return context;
}