iphone - 在 Core Graphics 中创建拼贴的多个剪切矩形

Question

我正在创建一个结合其他图像元素的拼贴画。这里有一些 ASCII 艺术来解释我在做什么:

Given images A, B and C,
AAA, BBB, CCC
AAA, BBB, CCC
AAA, BBB, CCC

I take part of A, part of B and part of C as columns:

Axx, xBx, xxC
Axx, xBx, xxC
Axx, xBx, xxC

...and combine them in one image like this:

ABC
ABC
ABC

where the first 1/3rd of the image is a colum of A's pic, the middle is a column of B's pic and the last is a column of C's pic.

我写了一些代码，但它只显示第一列而不显示其余部分……我想我必须以某种方式清除剪辑，但我不确定该怎么做，或者这是否是最好的方法。

+ (UIImage *)collageWithSize:(NSInteger)size fromImages:(NSArray *)images {
    NSMutableArray *selectedImages = [NSMutableArray array];
    [selectedImages addObjectsFromArray:images];

    // use the selectedImages for generating the thumbnail
    float columnWidth = (float)size/(float)[selectedImages count];

    //create a context to do our clipping in
    UIGraphicsBeginImageContext(CGSizeMake(size, size));
    CGContextRef currentContext = UIGraphicsGetCurrentContext();

    for (int i = 0; i < [selectedImages count]; i++) {
        // get the current image
        UIImage *image = [selectedImages objectAtIndex:i];

        //create a rect with the size we want to crop the image to
        CGRect clippedRect = CGRectMake(i*columnWidth, 0, columnWidth, size);
        CGContextClipToRect(currentContext, clippedRect);

        //create a rect equivalent to the full size of the image
        CGRect drawRect = CGRectMake(0, 0, size, size);

        //draw the image to our clipped context using our offset rect
        CGContextDrawImage(currentContext, drawRect, image.CGImage);
    }

    //pull the image from our cropped context
    UIImage *collage = UIGraphicsGetImageFromCurrentImageContext();

    //pop the context to get back to the default
    UIGraphicsEndImageContext();

    //Note: this is autoreleased
    return collage;
}

我做错了什么？

PS 图像也被颠倒了。

Answer 1

CGContextClipToRect 将当前剪辑矩形与提供的参数相交。因此，第二次调用它时，实际上是将剪裁区域变为空。

如果不恢复图形状态，就无法恢复裁剪区域。因此，在循环顶部调用 CGContextSaveGState，在循环底部调用 CGContextRestoreGState。

上下颠倒的部分可以通过调整当前的变换矩阵来修复:调用CGContextTranslateCTM移动原点，然后调用CGContextScaleCTM翻转y轴。