如何自动裁剪 UIImage？

Question

I have a UIImage which contains a shape; the rest is transparent. I'd like to get another UIImage by cropping out as much of the transparent part as possible, still retaining all of the non-transparent pixels - similar to the autocrop function in GIMP. How would I go about doing this?

Answer 1

This approach may be a little more invasive than what you were hoping for, but it gets the job done. What I'm doing is creating a bitmap context for the UIImage, obtaining a pointer to the raw image data, then sifting through it looking for non-transparent pixels. My method returns a CGRect which I use to create a new UIImage.

- (CGRect)cropRectForImage:(UIImage *)image {

CGImageRef cgImage = image.CGImage;
CGContextRef context = [self createARGBBitmapContextFromImage:cgImage];
if (context == NULL) return CGRectZero; 

size_t width = CGImageGetWidth(cgImage);
size_t height = CGImageGetHeight(cgImage);
CGRect rect = CGRectMake(0, 0, width, height);

CGContextDrawImage(context, rect, cgImage);

unsigned char *data = CGBitmapContextGetData(context);
CGContextRelease(context);

//Filter through data and look for non-transparent pixels.
int lowX = width;
int lowY = height;
int highX = 0;
int highY = 0;
if (data != NULL) {
    for (int y=0; y<height; y++) {
        for (int x=0; x<width; x++) {
            int pixelIndex = (width * y + x) * 4 /* 4 for A, R, G, B */;
            if (data[pixelIndex] != 0) { //Alpha value is not zero; pixel is not transparent.
                if (x < lowX) lowX = x;
                if (x > highX) highX = x;
                if (y < lowY) lowY = y;
                if (y > highY) highY = y;
            }
        }
    }
    free(data);
} else {
    return CGRectZero;
}

return CGRectMake(lowX, lowY, highX-lowX, highY-lowY);
}

The method to create the Bitmap Context:

- (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 width = CGImageGetWidth(inImage);
size_t height = 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 = (width * 4);
bitmapByteCount = (bitmapBytesPerRow * height);

// Use the generic RGB color space.
colorSpace = CGColorSpaceCreateDeviceRGB();
if (colorSpace == NULL) 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)
{
    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,
                                 width,
                                 height,
                                 8,      // bits per component
                                 bitmapBytesPerRow,
                                 colorSpace,
                                 kCGImageAlphaPremultipliedFirst);
if (context == NULL) free (bitmapData);

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

return context;
}

And finally, get your new cropped UIImage from the returned CGRect:

CGRect newRect = [self cropRectForImage:oldImage];
CGImageRef imageRef = CGImageCreateWithImageInRect(oldImage.CGImage, newRect);
UIImage *newImage = [UIImage imageWithCGImage:imageRef];
CGImageRelease(imageRef);

I grabbed a bit of that code from this very useful article . Hope it helps!

Answer 2

Swift Version:

extension UIImage {
func cropRect() -> CGRect {
    let cgImage = self.CGImage!
    let context = createARGBBitmapContextFromImage(cgImage)
    if context == nil {
        return CGRectZero
    }

    let height = CGFloat(CGImageGetHeight(cgImage))
    let width = CGFloat(CGImageGetWidth(cgImage))

    let rect = CGRectMake(0, 0, width, height)
    CGContextDrawImage(context, rect, cgImage)

    let data = UnsafePointer<CUnsignedChar>(CGBitmapContextGetData(context))

    if data == nil {
        return CGRectZero
    }

    var lowX = width
    var lowY = height
    var highX: CGFloat = 0
    var highY: CGFloat = 0

    //Filter through data and look for non-transparent pixels.
    for (var y: CGFloat = 0 ; y < height ; y++) {
        for (var x: CGFloat = 0; x < width ; x++) {
            let pixelIndex = (width * y + x) * 4 /* 4 for A, R, G, B */

            if data[Int(pixelIndex)] != 0 { //Alpha value is not zero pixel is not transparent.
                if (x < lowX) {
                    lowX = x
                }
                if (x > highX) {
                    highX = x
                }
                if (y < lowY) {
                    lowY = y
                }
                if (y > highY) {
                    highY = y
                }
            }
        }
    }


    return CGRectMake(lowX, lowY, highX-lowX, highY-lowY)
}
}

The method to create the Bitmap Context:

func createARGBBitmapContextFromImage(inImage: CGImageRef) -> CGContextRef? {

    let width = CGImageGetWidth(inImage)
    let height = CGImageGetHeight(inImage)

    let bitmapBytesPerRow = width * 4
    let bitmapByteCount = bitmapBytesPerRow * height

    let colorSpace = CGColorSpaceCreateDeviceRGB()
    if colorSpace == nil {
        return nil
    }

    let bitmapData = malloc(bitmapByteCount)
    if bitmapData == nil {
        return nil
    }

    let context = CGBitmapContextCreate (bitmapData,
        width,
        height,
        8,      // bits per component
        bitmapBytesPerRow,
        colorSpace,
        CGImageAlphaInfo.PremultipliedFirst.rawValue)

    return context
}

And finally, get your new cropped UIImage from the returned CGRect:

let image = // UIImage Source
let newRect = image.cropRect()
if let imageRef = CGImageCreateWithImageInRect(image.CGImage!, newRect) {
    let newImage = UIImage(CGImage: imageRef)
    // Use this new Image
}

Answer 3

Improved from @Danny182's answer, i also added white-space (any pixels brighter than 0xe0e0e0) trimming for my own need.

Usage:

let newimage = UIImage(named: "XXX")!.trim()

---

import UIKit

extension UIImage {

    func trim() -> UIImage {
        let newRect = self.cropRect
        if let imageRef = self.cgImage!.cropping(to: newRect) {
            return UIImage(cgImage: imageRef)
        }
        return self
    }

    var cropRect: CGRect {
        let cgImage = self.cgImage
        let context = createARGBBitmapContextFromImage(inImage: cgImage!)
        if context == nil {
            return CGRect.zero
        }

        let height = CGFloat(cgImage!.height)
        let width = CGFloat(cgImage!.width)

        let rect = CGRect(x: 0, y: 0, width: width, height: height)
        context?.draw(cgImage!, in: rect)

        //let data = UnsafePointer<CUnsignedChar>(CGBitmapContextGetData(context))
        guard let data = context?.data?.assumingMemoryBound(to: UInt8.self) else {
            return CGRect.zero
        }

        var lowX = width
        var lowY = height
        var highX: CGFloat = 0
        var highY: CGFloat = 0

        let heightInt = Int(height)
        let widthInt = Int(width)
        //Filter through data and look for non-transparent pixels.
        for y in (0 ..< heightInt) {
            let y = CGFloat(y)
            for x in (0 ..< widthInt) {
                let x = CGFloat(x)
                let pixelIndex = (width * y + x) * 4 /* 4 for A, R, G, B */

                if data[Int(pixelIndex)] == 0  { continue } // crop transparent

                if data[Int(pixelIndex+1)] > 0xE0 && data[Int(pixelIndex+2)] > 0xE0 && data[Int(pixelIndex+3)] > 0xE0 { continue } // crop white

                if (x < lowX) {
                    lowX = x
                }
                if (x > highX) {
                    highX = x
                }
                if (y < lowY) {
                    lowY = y
                }
                if (y > highY) {
                    highY = y
                }

            }
        }

        return CGRect(x: lowX, y: lowY, width: highX - lowX, height: highY - lowY)
    }

    func createARGBBitmapContextFromImage(inImage: CGImage) -> CGContext? {

        let width = inImage.width
        let height = inImage.height

        let bitmapBytesPerRow = width * 4
        let bitmapByteCount = bitmapBytesPerRow * height

        let colorSpace = CGColorSpaceCreateDeviceRGB()

        let bitmapData = malloc(bitmapByteCount)
        if bitmapData == nil {
            return nil
        }

        let context = CGContext (data: bitmapData,
                                 width: width,
                                 height: height,
                                 bitsPerComponent: 8,      // bits per component
            bytesPerRow: bitmapBytesPerRow,
            space: colorSpace,
            bitmapInfo: CGImageAlphaInfo.premultipliedFirst.rawValue)

        return context
    }
}

Answer 4

Swift 4

extension UIImage {
    
    func cropAlpha() -> UIImage {
        
        let cgImage = self.cgImage!;
        
        let width = cgImage.width
        let height = cgImage.height
        
        let colorSpace = CGColorSpaceCreateDeviceRGB()
        let bytesPerPixel:Int = 4
        let bytesPerRow = bytesPerPixel * width
        let bitsPerComponent = 8
        let bitmapInfo: UInt32 = CGImageAlphaInfo.premultipliedLast.rawValue | CGBitmapInfo.byteOrder32Big.rawValue
        
        guard let context = CGContext(data: nil, width: width, height: height, bitsPerComponent: bitsPerComponent, bytesPerRow: bytesPerRow, space: colorSpace, bitmapInfo: bitmapInfo),
            let ptr = context.data?.assumingMemoryBound(to: UInt8.self) else {
                return self
        }
        
        context.draw(self.cgImage!, in: CGRect(x: 0, y: 0, width: width, height: height))
        
        var minX = width
        var minY = height
        var maxX: Int = 0
        var maxY: Int = 0
        
        for x in 1 ..< width {
            for y in 1 ..< height {
                
                let i = bytesPerRow * Int(y) + bytesPerPixel * Int(x)
                let a = CGFloat(ptr[i + 3]) / 255.0
                
                if(a>0) {
                    if (x < minX) { minX = x };
                    if (x > maxX) { maxX = x };
                    if (y < minY) { minY = y};
                    if (y > maxY) { maxY = y};
                }
            }
        }
        
        let rect = CGRect(x: CGFloat(minX),y: CGFloat(minY), width: CGFloat(maxX-minX), height: CGFloat(maxY-minY))
        let imageScale:CGFloat = self.scale
        let croppedImage =  self.cgImage!.cropping(to: rect)!
        let ret = UIImage(cgImage: croppedImage, scale: imageScale, orientation: self.imageOrientation)
        
        return ret;
    }
    
}

Answer 5

I present you here the version of Swift 4. In addition to the code above, everybody forget context.clear() !!

func cropRect() -> CGRect {
    let cgImage = self.cgImage!

    let bitmapBytesPerRow = cgImage.width * 4
    let bitmapByteCount = bitmapBytesPerRow * cgImage.height
    let colorSpace = CGColorSpaceCreateDeviceRGB()
    let bitmapData = malloc(bitmapByteCount)

    if bitmapData == nil {
        return CGRect(x: 0, y: 0, width: 0, height: 0)
    }

    let context = CGContext (data: bitmapData, width: cgImage.width, height: cgImage.height, bitsPerComponent: 8,bytesPerRow: bitmapBytesPerRow,space: colorSpace,bitmapInfo: CGImageAlphaInfo.premultipliedFirst.rawValue)

    if context == nil {
        return CGRect(x: 0, y: 0, width: 0, height: 0)
    }

    let height = cgImage.height
    let width = cgImage.width

    let rect = CGRect(x: 0, y: 0, width: width, height: height)
    context?.clear(rect)
    context?.draw(cgImage, in: rect)

    let data = context?.data

    if data == nil {
        return CGRect(x: 0, y: 0, width: 0, height: 0)
    }

    var lowX = width
    var lowY = height
    var highX: Int = 0
    var highY: Int = 0

    //Filter through data and look for non-transparent pixels.
    for y in 0..<height {
        for x in  0..<width {
            let pixelIndex = (width * y + x) * 4 /* 4 for A, R, G, B */
            let color = data!.load(fromByteOffset: pixelIndex, as: UInt32.self)

            if color != 0 { //Alpha value is not zero pixel is not transparent.
                if (x < lowX) {
                    lowX = x
                }
                if (x > highX) {
                    highX = x
                }
                if (y < lowY) {
                    lowY = y
                }
                if (y > highY) {
                    highY = y
                }
            }
        }
    }


    return CGRect(x: lowX, y: lowY, width: highX-lowX, height: highY-lowY)
}

For getting the cropped image, just usual:

UIImage(cgImage: image.cgImage!.cropping(to: croppedRect)!)

Answer 6

Swift 3 (it's not a UIImage extension, I needed it in another class) it may save time for someone:

class EditImage {

static func cropRect(_ image: UIImage) -> CGRect {
    let cgImage = image.cgImage
    let context = createARGBBitmapContextFromImage(inImage: cgImage!)
    if context == nil {
        return CGRect.zero
    }

    let height = CGFloat(cgImage!.height)
    let width = CGFloat(cgImage!.width)

    let rect = CGRect(x: 0, y: 0, width: width, height: height)
    context?.draw(cgImage!, in: rect)

    //let data = UnsafePointer<CUnsignedChar>(CGBitmapContextGetData(context))
    let data = context?.data?.assumingMemoryBound(to: UInt8.self)

    if data == nil {
        return CGRect.zero
    }

    var lowX = width
    var lowY = height
    var highX: CGFloat = 0
    var highY: CGFloat = 0

    let heightInt = Int(height)
    let widthInt = Int(width)
    //Filter through data and look for non-transparent pixels.
    for y in (0 ..< heightInt) {
        let y = CGFloat(y)
        for x in (0 ..< widthInt) {
            let x = CGFloat(x)
            let pixelIndex = (width * y + x) * 4 /* 4 for A, R, G, B */

            if data?[Int(pixelIndex)] != 0 { //Alpha value is not zero pixel is not transparent.
                if (x < lowX) {
                    lowX = x
                }
                if (x > highX) {
                    highX = x
                }
                if (y < lowY) {
                    lowY = y
                }
                if (y > highY) {
                    highY = y
                }
            }
        }
    }


    return CGRect(x: lowX, y: lowY, width: highX - lowY, height: highY - lowY)
}

static func createARGBBitmapContextFromImage(inImage: CGImage) -> CGContext? {

    let width = inImage.width
    let height = inImage.height

    let bitmapBytesPerRow = width * 4
    let bitmapByteCount = bitmapBytesPerRow * height

    let colorSpace = CGColorSpaceCreateDeviceRGB()
    if colorSpace == nil {
        return nil
    }

    let bitmapData = malloc(bitmapByteCount)
    if bitmapData == nil {
        return nil
    }

    let context = CGContext (data: bitmapData,
        width: width,
        height: height,
        bitsPerComponent: 8,      // bits per component
        bytesPerRow: bitmapBytesPerRow,
        space: colorSpace,
    bitmapInfo: CGImageAlphaInfo.premultipliedFirst.rawValue)

    return context
}
}