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[Swift 5] NSImage/UIImage Crop/Trim Transparency
// Image+Trim.swift
//
// Copyright © 2020 Christopher Zielinski.
// https://gist.github.com/chriszielinski/aec9a2f2ba54745dc715dd55f5718177
//
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in
// all copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
// THE SOFTWARE.
#if canImport(UIKit)
import UIKit
#else
import AppKit
#endif
#if canImport(UIKit)
typealias Image = UIImage
#else
typealias Image = NSImage
#endif
extension Image {
/// Crops the insets of transparency around the image.
///
/// - Parameters:
/// - maximumAlphaChannel: The maximum alpha channel value to consider _transparent_ and thus crop. Any alpha value
/// strictly greater than `maximumAlphaChannel` will be considered opaque.
func trimmingTransparentPixels(maximumAlphaChannel: UInt8 = 0) -> Image? {
guard size.height > 1 && size.width > 1
else { return self }
#if canImport(UIKit)
guard let cgImage = cgImage?.trimmingTransparentPixels(maximumAlphaChannel: maximumAlphaChannel)
else { return nil }
return UIImage(cgImage: cgImage, scale: scale, orientation: imageOrientation)
#else
guard let cgImage = cgImage(forProposedRect: nil, context: nil, hints: nil)?
.trimmingTransparentPixels(maximumAlphaChannel: maximumAlphaChannel)
else { return nil }
let scale = recommendedLayerContentsScale(0)
let scaledSize = CGSize(width: CGFloat(cgImage.width) / scale,
height: CGFloat(cgImage.height) / scale)
let image = NSImage(cgImage: cgImage, size: scaledSize)
image.isTemplate = isTemplate
return image
#endif
}
}
extension CGImage {
/// Crops the insets of transparency around the image.
///
/// - Parameters:
/// - maximumAlphaChannel: The maximum alpha channel value to consider _transparent_ and thus crop. Any alpha value
/// strictly greater than `maximumAlphaChannel` will be considered opaque.
func trimmingTransparentPixels(maximumAlphaChannel: UInt8 = 0) -> CGImage? {
let trimmer = _CGImageTransparencyTrimmer(image: self, maximumAlphaChannel: maximumAlphaChannel)
let image = trimmer?.trim()
free(trimmer?.zeroByteBlock)
return image
}
}
private struct _CGImageTransparencyTrimmer {
let image: CGImage
let maximumAlphaChannel: UInt8
let cgContext: CGContext
let zeroByteBlock: UnsafeMutableRawPointer
let pixelRowRange: Range<Int>
let pixelColumnRange: Range<Int>
init?(image: CGImage, maximumAlphaChannel: UInt8) {
guard let cgContext = CGContext(data: nil,
width: image.width,
height: image.height,
bitsPerComponent: 8,
bytesPerRow: 0,
space: CGColorSpaceCreateDeviceGray(),
bitmapInfo: CGImageAlphaInfo.alphaOnly.rawValue),
cgContext.data != nil
else { return nil }
cgContext.draw(image,
in: CGRect(origin: .zero,
size: CGSize(width: image.width,
height: image.height)))
guard let zeroByteBlock = calloc(image.width, MemoryLayout<UInt8>.size)
else { return nil }
self.image = image
self.maximumAlphaChannel = maximumAlphaChannel
self.cgContext = cgContext
self.zeroByteBlock = zeroByteBlock
pixelRowRange = 0..<image.height
pixelColumnRange = 0..<image.width
}
func trim() -> CGImage? {
guard let topInset = firstOpaquePixelRow(in: pixelRowRange),
let bottomOpaqueRow = firstOpaquePixelRow(in: pixelRowRange.reversed()),
let leftInset = firstOpaquePixelColumn(in: pixelColumnRange),
let rightOpaqueColumn = firstOpaquePixelColumn(in: pixelColumnRange.reversed())
else { return nil }
let bottomInset = (image.height - 1) - bottomOpaqueRow
let rightInset = (image.width - 1) - rightOpaqueColumn
guard !(topInset == 0 && bottomInset == 0 && leftInset == 0 && rightInset == 0)
else { return image }
return image.cropping(to: CGRect(origin: CGPoint(x: leftInset, y: topInset),
size: CGSize(width: image.width - (leftInset + rightInset),
height: image.height - (topInset + bottomInset))))
}
@inlinable
func isPixelOpaque(column: Int, row: Int) -> Bool {
// Sanity check: It is safe to get the data pointer in iOS 4.0+ and macOS 10.6+ only.
assert(cgContext.data != nil)
return cgContext.data!.load(fromByteOffset: (row * cgContext.bytesPerRow) + column, as: UInt8.self)
> maximumAlphaChannel
}
@inlinable
func isPixelRowTransparent(_ row: Int) -> Bool {
assert(cgContext.data != nil)
// `memcmp` will efficiently check if the entire pixel row has zero alpha values
return memcmp(cgContext.data! + (row * cgContext.bytesPerRow), zeroByteBlock, image.width) == 0
// When the entire row is NOT zeroed, we proceed to check each pixel's alpha
// value individually until we locate the first "opaque" pixel (very ~not~ efficient).
|| !pixelColumnRange.contains(where: { isPixelOpaque(column: $0, row: row) })
}
@inlinable
func firstOpaquePixelRow<T: Sequence>(in rowRange: T) -> Int? where T.Element == Int {
return rowRange.first(where: { !isPixelRowTransparent($0) })
}
@inlinable
func firstOpaquePixelColumn<T: Sequence>(in columnRange: T) -> Int? where T.Element == Int {
return columnRange.first(where: { column in
pixelRowRange.contains(where: { isPixelOpaque(column: column, row: $0) })
})
}
}
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