Mac OS X 10.7.4
我正在绘制通过+[NSGraphicsContext graphicsContextWithBitmapImageRep:]
创建的屏幕外图形上下文。
当我使用NSBezierPath
类绘制到这个图形上下文时,一切都按预期工作。
但是,当我使用CGContextRef
C函数绘制到这个图形上下文时,我看不到我的绘图结果。什么都行不通。
由于我不会进入的原因,我真的需要使用CGContextRef
函数(而不是Cocoa NSBezierPath
类)。
我的代码示例如下所示。我试图画一个简单的“X”。一次使用NSBezierPath
,一次使用CGContextRef
C函数。第一笔中风起作用,第二笔中风不起作用。我究竟做错了什么?
NSRect imgRect = NSMakeRect(0.0, 0.0, 100.0, 100.0);
NSSize imgSize = imgRect.size;
NSBitmapImageRep *offscreenRep = [[[NSBitmapImageRep alloc]
initWithBitmapDataPlanes:NULL
pixelsWide:imgSize.width
pixelsHigh:imgSize.height
bitsPerSample:8
samplesPerPixel:4
hasAlpha:YES
isPlanar:NO
colorSpaceName:NSDeviceRGBColorSpace
bitmapFormat:NSAlphaFirstBitmapFormat
bytesPerRow:0
bitsPerPixel:0] autorelease];
// set offscreen context
NSGraphicsContext *g = [NSGraphicsContext graphicsContextWithBitmapImageRep:offscreenRep];
[NSGraphicsContext setCurrentContext:g];
NSImage *img = [[[NSImage alloc] initWithSize:imgSize] autorelease];
CGContextRef ctx = [g graphicsPort];
// lock and draw
[img lockFocus];
// draw first stroke with Cocoa. this works!
NSPoint p1 = NSMakePoint(NSMaxX(imgRect), NSMinY(imgRect));
NSPoint p2 = NSMakePoint(NSMinX(imgRect), NSMaxY(imgRect));
[NSBezierPath strokeLineFromPoint:p1 toPoint:p2];
// draw second stroke with Core Graphics. This doesn't work!
CGContextBeginPath(ctx);
CGContextMoveToPoint(ctx, 0.0, 0.0);
CGContextAddLineToPoint(ctx, imgSize.width, imgSize.height);
CGContextClosePath(ctx);
CGContextStrokePath(ctx);
[img unlockFocus];
您没有指定查看结果的方式。我假设你正在看NSImage
img
而不是NSBitmapImageRep
offscreenRep
。
当你调用[img lockFocus]
时,你正在改变当前的NSGraphicsContext
作为绘制到img
的上下文。所以,NSBezierPath
绘图进入img
,这就是你所看到的。 CG绘图进入你不看的offscreenRep
。
不是将焦点锁定到NSImage并绘制到NSImage中,而是创建一个NSImage并将offscreenRep添加为其代表之一。
NSRect imgRect = NSMakeRect(0.0, 0.0, 100.0, 100.0);
NSSize imgSize = imgRect.size;
NSBitmapImageRep *offscreenRep = [[[NSBitmapImageRep alloc]
initWithBitmapDataPlanes:NULL
pixelsWide:imgSize.width
pixelsHigh:imgSize.height
bitsPerSample:8
samplesPerPixel:4
hasAlpha:YES
isPlanar:NO
colorSpaceName:NSDeviceRGBColorSpace
bitmapFormat:NSAlphaFirstBitmapFormat
bytesPerRow:0
bitsPerPixel:0] autorelease];
// set offscreen context
NSGraphicsContext *g = [NSGraphicsContext graphicsContextWithBitmapImageRep:offscreenRep];
[NSGraphicsContext saveGraphicsState];
[NSGraphicsContext setCurrentContext:g];
// draw first stroke with Cocoa
NSPoint p1 = NSMakePoint(NSMaxX(imgRect), NSMinY(imgRect));
NSPoint p2 = NSMakePoint(NSMinX(imgRect), NSMaxY(imgRect));
[NSBezierPath strokeLineFromPoint:p1 toPoint:p2];
// draw second stroke with Core Graphics
CGContextRef ctx = [g graphicsPort];
CGContextBeginPath(ctx);
CGContextMoveToPoint(ctx, 0.0, 0.0);
CGContextAddLineToPoint(ctx, imgSize.width, imgSize.height);
CGContextClosePath(ctx);
CGContextStrokePath(ctx);
// done drawing, so set the current context back to what it was
[NSGraphicsContext restoreGraphicsState];
// create an NSImage and add the rep to it
NSImage *img = [[[NSImage alloc] initWithSize:imgSize] autorelease];
[img addRepresentation:offscreenRep];
// then go on to save or view the NSImage
我想知道为什么每个人都写这么复杂的代码来绘制图像。除非你关心图像的确切位图表示(通常你不关心!),否则不需要创建一个。您可以创建一个空白图像并直接绘制到它。在这种情况下,系统将创建适当的位图表示(或者可能是PDF表示或系统认为更适合绘图的任何内容)。
init方法的文档
- (instancetype)initWithSize:(NSSize)aSize
从MacOS 10.0开始存在但仍未被弃用,清楚地说:
使用此方法初始化图像对象后,您需要在尝试绘制图像之前提供图像内容。您可以将焦点锁定在图像上并绘制到图像,也可以显式添加您创建的图像表示。
所以这就是我编写代码的方式:
NSRect imgRect = NSMakeRect(0.0, 0.0, 100.0, 100.0);
NSImage * image = [[NSImage alloc] initWithSize:imgRect.size];
[image lockFocus];
// draw first stroke with Cocoa
NSPoint p1 = NSMakePoint(NSMaxX(imgRect), NSMinY(imgRect));
NSPoint p2 = NSMakePoint(NSMinX(imgRect), NSMaxY(imgRect));
[NSBezierPath strokeLineFromPoint:p1 toPoint:p2];
// draw second stroke with Core Graphics
CGContextRef ctx = [[NSGraphicsContext currentContext] graphicsPort];
CGContextBeginPath(ctx);
CGContextMoveToPoint(ctx, 0.0, 0.0);
CGContextAddLineToPoint(ctx, imgRect.size.width, imgRect.size.height);
CGContextClosePath(ctx);
CGContextStrokePath(ctx);
[image unlockFocus];
这就是所有人。
graphicsPort
实际上是void *
:
@property (readonly) void * graphicsPort
并记录为
由图形端口表示的低级,特定于平台的图形上下文。
这可能是一切,但最后的说明
在OS X中,这是Core Graphics上下文,一个
CGContextRef
对象(opaque类型)。
此属性已在10.10中弃用,以支持新属性
@property (readonly) CGContextRef CGContext
仅适用于10.10及更高版本。如果你必须支持旧系统,仍然可以使用graphicsPort
。
这是绘制相同图像的3种方法(Swift 4)。
@Mecki建议的方法生成没有blurring
伪像(如模糊曲线)的图像。但这可以通过调整CGContext
设置(本例中未包含)来修复。
public struct ImageFactory {
public static func image(size: CGSize, fillColor: NSColor, rounded: Bool = false) -> NSImage? {
let rect = CGRect(x: 0, y: 0, width: size.width, height: size.height)
return drawImage(size: size) { context in
if rounded {
let radius = min(size.height, size.width)
let path = NSBezierPath(roundedRect: rect, xRadius: 0.5 * radius, yRadius: 0.5 * radius).cgPath
context.addPath(path)
context.clip()
}
context.setFillColor(fillColor.cgColor)
context.fill(rect)
}
}
}
extension ImageFactory {
private static func drawImage(size: CGSize, drawingCalls: (CGContext) -> Void) -> NSImage? {
return drawImageInLockedImageContext(size: size, drawingCalls: drawingCalls)
}
private static func drawImageInLockedImageContext(size: CGSize, drawingCalls: (CGContext) -> Void) -> NSImage? {
let image = NSImage(size: size)
image.lockFocus()
guard let context = NSGraphicsContext.current else {
image.unlockFocus()
return nil
}
drawingCalls(context.cgContext)
image.unlockFocus()
return image
}
// Has scalling or antialiasing issues, like blurred curves.
private static func drawImageInBitmapImageContext(size: CGSize, drawingCalls: (CGContext) -> Void) -> NSImage? {
guard let offscreenRep = NSBitmapImageRep(pixelsWide: Int(size.width), pixelsHigh: Int(size.height),
bitsPerSample: 8, samplesPerPixel: 4, hasAlpha: true,
isPlanar: false, colorSpaceName: .deviceRGB) else {
return nil
}
guard let context = NSGraphicsContext(bitmapImageRep: offscreenRep) else {
return nil
}
NSGraphicsContext.saveGraphicsState()
NSGraphicsContext.current = context
drawingCalls(context.cgContext)
NSGraphicsContext.restoreGraphicsState()
let img = NSImage(size: size)
img.addRepresentation(offscreenRep)
return img
}
// Has scalling or antialiasing issues, like blurred curves.
private static func drawImageInCGContext(size: CGSize, drawingCalls: (CGContext) -> Void) -> NSImage? {
let colorSpace = CGColorSpaceCreateDeviceRGB()
let bitmapInfo = CGBitmapInfo(rawValue: CGImageAlphaInfo.premultipliedLast.rawValue)
guard let context = CGContext(data: nil, width: Int(size.width), height: Int(size.height), bitsPerComponent: 8,
bytesPerRow: 0, space: colorSpace, bitmapInfo: bitmapInfo.rawValue) else {
return nil
}
drawingCalls(context)
guard let image = context.makeImage() else {
return nil
}
return NSImage(cgImage: image, size: size)
}
}
Swift 4:我使用这段代码,从UIKit复制方便的API(但在macOS上运行):
public class UIGraphicsImageRenderer {
let size: CGSize
init(size: CGSize) {
self.size = size
}
func image(actions: (CGContext) -> Void) -> NSImage {
let image = NSImage(size: size)
image.lockFocusFlipped(true)
actions(NSGraphicsContext.current!.cgContext)
image.unlockFocus()
return image
}
}
用法:
let renderer = UIGraphicsImageRenderer(size: imageSize)
let image = renderer.image { ctx in
// Drawing commands here
}