Merging two UIImages faster than CGContextDrawImage - iphone

Question

I'm merging two UIImages into one context. It works, but it performs pretty slow and I'm in the need of a faster solution. As my solution is it takes about 400ms to make the mergeImage: withImage: call on an iPad 1G.
Here's what I do:
-(CGContextRef)mergeImage:(UIImage*)img1 withImage:(UIImage*)img2
{
CGSize size = [ImageToolbox getScreenSize];
CGContextRef context = [ImageToolbox createARGBBitmapContextFromImageSize:CGSizeMake(size.width, size.height)];
CGContextSetRenderingIntent(context, kCGRenderingIntentSaturation);
CGContextDrawImage(context, CGRectMake(0, 0, size.width, size.height), img1.CGImage);
CGContextDrawImage(context, CGRectMake(0, 0, size.width, size.height), img2.CGImage);
return context;
}
And here's the static methods from the ImageToolbox class:
static CGRect screenRect;
+ (CGContextRef)createARGBBitmapContextFromImageSize:(CGSize)imageSize
{
CGContextRef context = NULL;
CGColorSpaceRef colorSpace;
void * bitmapData;
int bitmapByteCount;
int bitmapBytesPerRow;
size_t pixelsWide = imageSize.width;
size_t pixelsHigh = imageSize.height;
bitmapBytesPerRow = (pixelsWide * 4);
bitmapByteCount = (bitmapBytesPerRow * pixelsHigh);
colorSpace = CGColorSpaceCreateDeviceRGB();
if (colorSpace == NULL)
{
fprintf(stderr, "Error allocating color space\n");
return NULL;
}
bitmapData = malloc( bitmapByteCount );
if (bitmapData == NULL)
{
fprintf (stderr, "Memory not allocated!");
CGColorSpaceRelease( colorSpace );
return NULL;
}
context = CGBitmapContextCreate (bitmapData,
pixelsWide,
pixelsHigh,
8, // bits per component
bitmapBytesPerRow,
colorSpace,
kCGImageAlphaPremultipliedFirst);
if (context == NULL)
{
free (bitmapData);
fprintf (stderr, "Context not created!");
}
CGColorSpaceRelease( colorSpace );
return context;
}
+(CGSize)getScreenSize
{
if (screenRect.size.width == 0 && screenRect.size.height == 0)
{
screenRect = [[UIScreen mainScreen] bounds];
}
return CGSizeMake(screenRect.size.height, screenRect.size.width-20);
}
Any suggestions to increase the performance?

Answer 1

I would definitely recommend using Instruments to profile what message is taking the most time so you can really break it down. Also, I have written a couple methods which I think should do the same thing with a lot less code, but you must have everything written out the way you do to really keep things customizable. Here they are anyways:
-(CGContextRef)mergeImage:(UIImage *)img1 withImage:(UIImage *)img2
{
CGSize size = [ImageToolbox getScreenSize];
CGRect rect = CGRectMake(0, 0, size.width, size.height);
UIGraphicsBeginImageContextWithOptions(size, YES, 1.0);
CGContextRef context = UIGraphicsGetCurrentContext();
CGContextSetRenderingIntent(context, kCGRenderingIntentSaturation);
[img1 drawInRect:rect];
[img2 drawInRect:rect];
UIGraphicsEndImageContext();
return context;
}
Or if you wanted the combined image right away:
- (UIImage *)mergeImage:(UIImage *)img1 withImage:(UIImage *)img2
{
CGSize size = [ImageToolbox getScreenSize];
CGRect rect = CGRectMake(0, 0, size.width, size.height);
UIGraphicsBeginImageContextWithOptions(size, YES, 1.0);
CGContextRef context = UIGraphicsGetCurrentContext();
CGContextSetRenderingIntent(context, kCGRenderingIntentSaturation);
[img1 drawInRect:rect];
[img2 drawInRect:rect];
UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
return image;
}
I have no idea if they would be faster or not, but I really wouldn't know how to speed up what you have very easily unless you had the instruments profile break down.
In any case, I hope this helps.

Answer 2

I did no manage to find a faster way of merging the images. I reduced the images sizes to make the operation faster.