检测具有透明背景的 UIImage 的裁剪矩形
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【中文标题】检测具有透明背景的 UIImage 的裁剪矩形【英文标题】:Detect cropping rectangle for UIImage with transparent background 【发布时间】:2013-05-15 15:32:35 【问题描述】:采用透明背景的 UIImage 并确定要裁剪到的最小矩形以便只留下可见图像数据的策略是什么(当然,如果图像数据不是矩形,则还有额外的透明背景)?
我发现了很多关于将 UIImage 裁剪为 CGRect 的信息,大量需要用户干预的裁剪视图控制器,以及几个具有图像处理类和类别的开源库(包括 MGImageUtilities 和 NYXImagesKit),但还没有解决的问题我的特殊问题。
我当前的应用程序面向 ios 5.0,因此与它的兼容性将是最佳的。
编辑:顺便说一句,我希望在寻找边缘边界的行和列中的图像数据的最坏情况下,有一种比蛮力查看每个像素更好的方法。
【问题讨论】:
【参考方案1】:你有机会见到https://gist.github.com/spinogrizz/3549921吗?
看起来正是你需要的。
为了不丢失,从该页面复制和粘贴:
- (UIImage *) imageByTrimmingTransparentPixels
int rows = self.size.height;
int cols = self.size.width;
int bytesPerRow = cols*sizeof(uint8_t);
if ( rows < 2 || cols < 2 )
return self;
//allocate array to hold alpha channel
uint8_t *bitmapData = calloc(rows*cols, sizeof(uint8_t));
//create alpha-only bitmap context
CGContextRef contextRef = CGBitmapContextCreate(bitmapData, cols, rows, 8, bytesPerRow, NULL, kCGImageAlphaOnly);
//draw our image on that context
CGImageRef cgImage = self.CGImage;
CGRect rect = CGRectMake(0, 0, cols, rows);
CGContextDrawImage(contextRef, rect, cgImage);
//summ all non-transparent pixels in every row and every column
uint16_t *rowSum = calloc(rows, sizeof(uint16_t));
uint16_t *colSum = calloc(cols, sizeof(uint16_t));
//enumerate through all pixels
for ( int row = 0; row < rows; row++)
for ( int col = 0; col < cols; col++)
if ( bitmapData[row*bytesPerRow + col] ) //found non-transparent pixel
rowSum[row]++;
colSum[col]++;
//initialize crop insets and enumerate cols/rows arrays until we find non-empty columns or row
UIEdgeInsets crop = UIEdgeInsetsMake(0, 0, 0, 0);
for ( int i = 0; i<rows; i++ ) //top
if ( rowSum[i] > 0 )
crop.top = i; break;
for ( int i = rows; i >= 0; i-- ) //bottom
if ( rowSum[i] > 0 )
crop.bottom = MAX(0, rows-i-1); break;
for ( int i = 0; i<cols; i++ ) //left
if ( colSum[i] > 0 )
crop.left = i; break;
for ( int i = cols; i >= 0; i-- ) //right
if ( colSum[i] > 0 )
crop.right = MAX(0, cols-i-1); break;
free(bitmapData);
free(colSum);
free(rowSum);
if ( crop.top == 0 && crop.bottom == 0 && crop.left == 0 && crop.right == 0 )
//no cropping needed
return self;
else
//calculate new crop bounds
rect.origin.x += crop.left;
rect.origin.y += crop.top;
rect.size.width -= crop.left + crop.right;
rect.size.height -= crop.top + crop.bottom;
//crop it
CGImageRef newImage = CGImageCreateWithImageInRect(cgImage, rect);
//convert back to UIImage
return [UIImage imageWithCGImage:newImage];
【讨论】:
不适合我——但希望有另一个版本在浮动......【参考方案2】:它在 Swift 4 中
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;
【讨论】:
@JsonCragun,输出图像模糊,图像中有小矩形。您在项目中使用上述代码吗?你面临这个问题吗?【参考方案3】:请参阅 WWDC 上的 CoreImage
幻灯片。
你的回答很直接。
【讨论】:
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