Core Graphics中精确的像素网格覆盖？

Question

在我创建以像素为中心的图像编辑器的实验中,我一直在尝试绘制精确的网格叠加,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,尤其是尺寸较小的网格.对于每几个普通列,这是一个略大的列的常规模式,所以我认为这是一个舍入问题,但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, 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？

Answer 1

更新:我设法让你的代码运行一些调整 - CGImageGetSize()来自哪里？ - 我无法真正看到问题,除了列甚至不是非常小的尺寸.这就是它必须如何工作.解决这个问题的唯一方法是将缩放比例固定为图像大小的整数倍 - 换句话说,获取小于视图大小的图像大小的最大整数倍 - 或者减少屏幕上绘制的线条数量小尺寸摆脱这个人工制品.只有在大多数编辑器中放大时,像素网格才会变得可见.更不用说如果网格仍然以3-4倍的分辨率可见,那么你的视图太忙了.

我无法运行你提供的代码,因为那里有一堆类ivars,但从粗略的一瞥,我会说它与绘制像素边界有关.在你舍入到一个整数以消除模糊的AA人工制品之后(我注意到你关闭了AA,但理想情况下你不应该这样做),然后你需要在你的原点上添加0.5来让你的线在中心绘制像素而不是边界.

像这样:

+---X---+---+---+---+---+
|   |   |   | Y |   |   |
+---+---+---+---+---+---+

X : CGPoint (1, 1)
Y : CGPoint (3.5, 0.5)

你想从像素的中心绘制,因为否则你的线跨越两个像素.

换句话说,在设置xPoints和yPoints的地方,确保使用floor()或round()你的值,然后加0.5.