如何解决Core Graphics中的精确像素网格叠加?
| 在创建以像素为中心的图像编辑器的实验中,我一直试图绘制精确的网格覆盖图,以帮助引导用户尝试访问某些像素。但是,我绘制的网格不是很均匀,特别是在较小的尺寸下。这是一种常规模式,每隔几列普通列都会增加一个稍大的列,因此我认为这是一个舍入问题,但是我在代码中看不到它。这是我的代码: - (void)drawRect:(NSRect)dirtyRect
{
context = [[NSGraphicsContext currentContext] graphicsPort];
CGContextAddRect(context,NSRectToCGRect(self.bounds));
CGContextSetRGBStrokeColor(context,1.0f,0.0f,1.0f);
CGContextStrokePath(context);
CGContextSetInterpolationQuality(context,kCGInterpolationNone);
CGContextSetShouldAntialias(context,NO);
if (image)
{
NSRect imageRect = NSZeroRect;
imageRect.size = CGImageGetSize([image CGImage]);
drawRect = [self bounds];
NSRect viewRect = drawRect;
CGFloat aspectRatio = imageRect.size.width / imageRect.size.height;
if (viewRect.size.width / viewRect.size.height <= aspectRatio)
{
drawRect.size.width = viewRect.size.width;
drawRect.size.height = imageRect.size.height * (viewRect.size.width / imageRect.size.width);
}
else
{
drawRect.size.height = viewRect.size.height;
drawRect.size.width = imageRect.size.width * (viewRect.size.height / imageRect.size.height);
}
drawRect.origin.x += (viewRect.size.width - drawRect.size.width) / 2.0;
drawRect.origin.y += (viewRect.size.height - drawRect.size.height) / 2.0;
CGContextDrawImage(context,drawRect,[image CGImage]);
if (showPixelGrid)
{
//Draw grid by creating start and end points for vertical and horizontal lines.
//FIXME: Grid is uneven,especially at smaller sizes.
CGContextSetStrokeColorWithColor(context,CGColorGetConstantColor(kCGColorBlack));
CGContextAddRect(context,drawRect);
CGContextStrokePath(context);
NSUInteger numXPoints = (NSUInteger)imageRect.size.width * 2;
NSUInteger numYPoints = (NSUInteger)imageRect.size.height * 2;
CGPoint xPoints[numXPoints];
CGPoint yPoints[numYPoints];
CGPoint startPoint;
CGPoint endPoint;
CGFloat widthRatio = drawRect.size.width / imageRect.size.width;
CGFloat heightRatio = drawRect.size.height / imageRect.size.height;
startPoint.x = drawRect.origin.x;
startPoint.y = drawRect.origin.y;
endPoint.x = drawRect.origin.x;
endPoint.y = drawRect.size.height + drawRect.origin.y;
for (NSUInteger i = 0; i < numXPoints; i += 2)
{
startPoint.x += widthRatio;
endPoint.x += widthRatio;
xPoints[i] = startPoint;
xPoints[i + 1] = endPoint;
}
startPoint.x = drawRect.origin.x;
startPoint.y = drawRect.origin.y;
endPoint.x = drawRect.size.width + drawRect.origin.x;
endPoint.y = drawRect.origin.y;
for (NSUInteger i = 0; i < numYPoints; i += 2)
{
startPoint.y += heightRatio;
endPoint.y += heightRatio;
yPoints[i] = startPoint;
yPoints[i + 1] = endPoint;
}
CGContextStrokeLineSegments(context,xPoints,numXPoints);
CGContextStrokeLineSegments(context,yPoints,numYPoints);
}
}
}
有任何想法吗?
解决方法
更新:我设法进行一些调整来运行您的代码-CGImageGetSize()来自何处? -而且我真的看不到问题,除了即使是很小的列,列也不是全部。这就是它必须如何工作的。解决此问题的唯一方法是将缩放比例固定为图像尺寸的整数倍-换句话说,获得小于图像尺寸的图像尺寸的最大整数倍-或以非常低的比例减少在屏幕上绘制的线数小尺寸,以摆脱这种人工制品。在大多数编辑器中长时间缩放时,像素网格才可见是有原因的。更不用说如果仍然可以3-4倍的分辨率看到网格,则说明视图太忙了。
我无法运行您提供的代码,因为其中有很多类ivars,但是从粗略地看,我会说这与在像素边界上绘制有关。四舍五入为整数以消除模糊的AA伪像后(我注意到您已关闭AA,但理想情况下不必这样做),然后需要在原点上加上0.5,以便在像素的中心,而不是边界。
像这样:
+---X---+---+---+---+---+
| | | | Y | | |
+---+---+---+---+---+---+
X : CGPoint (1,1)
Y : CGPoint (3.5,0.5)
您想从像素的中心进行绘制,因为否则,您的线会跨越两个像素。
换句话说,在设置xPoints和yPoints的位置,请确保将floor()或round()设置为您的值,然后添加0.5。
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