iOS Performance Tuning: fastest way to get pixel color for large images

There are a number of questions/answers regarding how to get the pixel color of an image for a given point. However, all of these answers are really slow (100-500ms) for large images (even as small as 1000 x 1300, for example).

Most of the code samples out there draw to an image context. All of them take time when the actual draw takes place:

CGContextDrawImage(context, CGRectMake(0.0f, 0.0f, (CGFloat)width, (CGFloat)height), cgImage)

Examining this in Instruments reveals that the draw is being done by copying the data from the source image:

enter image description here

I have even tried a different means of getting at the data, hoping that getting to the bytes themselves would actually prove much more efficient.

NSInteger pointX = trunc(point.x);
NSInteger pointY = trunc(point.y);
CGImageRef cgImage = CGImageCreateWithImageInRect(self.CGImage, 
                           CGRectMake(pointX * self.scale, 
                                      pointY * self.scale, 
                                      1.0f, 
                                      1.0f));

CGDataProviderRef provider = CGImageGetDataProvider(cgImage);
CFDataRef data = CGDataProviderCopyData(provider);

CGImageRelease(cgImage);

UInt8* buffer = (UInt8*)CFDataGetBytePtr(data);

CGFloat red   = (float)buffer[0] / 255.0f;
CGFloat green = (float)buffer[1] / 255.0f;
CGFloat blue  = (float)buffer[2] / 255.0f;
CGFloat alpha = (float)buffer[3] / 255.0f;

CFRelease(data);

UIColor *pixelColor = [UIColor colorWithRed:red green:green blue:blue alpha:alpha];

return pixelColor;

This method takes it's time on the data copy:

CFDataRef data = CGDataProviderCopyData(provider);

It would appear that it too is reading the data from disk, instead of the CGImage instance I am creating:

ios performance uiimage core-graphics

Edit Report

1 Answer

I have yet to find a way to get access to the drawn (in frame buffer) pixels. The fastest method I've measured is:

Indicate you want the image to be cached by specifying kCGImageSourceShouldCache when creating it.
(optional) Precache the image by forcing it to render.
Draw the image a 1x1 bitmap context.

The cost of this method is the cached bitmap, which may have a lifetime as long as the CGImage it is associated with. The code ends up looking something like this:

Create image w/ ShouldCache flag

NSDictionary *options = @{ (id)kCGImageSourceShouldCache: @(YES) };
CGImageSourceRef imageSource = CGImageSourceCreateWithData((__bridge CFDataRef)imageData, NULL);
CGImageRef cgimage = CGImageSourceCreateImageAtIndex(imageSource, 0, (__bridge CFDictionaryRef)options);
UIImage *image = [UIImage imageWithCGImage:cgimage];
CGImageRelease(cgimage);

Precache image

UIGraphicsBeginImageContext(CGSizeMake(1, 1));
[image drawAtPoint:CGPointZero];
UIGraphicsEndImageContext();

Draw image to a 1x1 bitmap context

unsigned char pixelData[] = { 0, 0, 0, 0 };
CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
CGContextRef context = CGBitmapContextCreate(pixelData, 1, 1, 8, 4, colorSpace, kCGImageAlphaPremultipliedLast | kCGBitmapByteOrder32Big);
CGImageRef cgimage = image.CGImage;
int imageWidth = CGImageGetWidth(cgimage);
int imageHeight = CGImageGetHeight(cgimage);
CGContextDrawImage(context, CGRectMake(-testPoint.x, testPoint.y - imageHeight, imageWidth, imageHeight), cgimage);
CGColorSpaceRelease(colorSpace);
CGContextRelease(context);

pixelData has the R, G, B, and A values of the pixel at testPoint.

answered 2012-09-11T17:24:08.070

iOS Performance Tuning: fastest way to get pixel color for large images

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