OpenCV模板匹配调用cv::Mat::create报错(-215:Assertion failed) s>=0如何解决

这个error: (-215:Assertion failed) s >= 0 in function 'setSize'是OpenCV创建矩阵时的参数合法性校验失败，触发原因有两个：

• 核心直接原因：result.create(result_cols, result_rows, CV_32FC1)传入的宽高参数为负数。模板匹配的结果矩阵宽高计算公式是大图宽 - 模板宽 + 1、大图高 - 模板高 + 1，如果待搜索的大图宽度/高度小于子图（模板图）的宽度/高度，计算出来的结果就会是负数，触发断言错误。
• 场景差异原因：App包内的图像资源都是提前处理好的，尺寸固定符合匹配要求；而设备拍摄的图像存在两个常见问题导致尺寸不符合要求：
  1. UIImage的imageOrientation属性未被正确处理：你调用的CVMat转换方法如果没有处理图像方向，竖拍的图像转换后会出现宽高颠倒的情况，导致实际大图的宽高小于子图。
  2. 拍摄的图像本身分辨率低于子图的分辨率，直接导致计算出来的结果矩阵宽高为负。

1. 增加前置合法性校验 #

在创建结果矩阵之前先校验尺寸是否合法，避免触发崩溃：

int result_cols =  tempMat1.cols - tempMat2.cols + 1;
int result_rows = tempMat1.rows - tempMat2.rows + 1;

// 新增校验逻辑
if (result_cols <= 0 || result_rows <= 0) {
    NSLog(@"待搜索图像尺寸小于子图，无法执行匹配");
    return [[UIImage alloc] initWithCVMat:tempMat1];
}

result.create( result_cols, result_rows, CV_32FC1);

2. 校正UIImage的方向 #

在把UIImage转换为Mat之前，先校正图像方向，确保转换后的宽高符合实际预期，你可以新增如下UIImage分类方法校正方向：

@implementation UIImage (OrientationFix)
- (UIImage *)fixOrientation {
    if (self.imageOrientation == UIImageOrientationUp) return self;
    CGAffineTransform transform = CGAffineTransformIdentity;
    switch (self.imageOrientation) {
        case UIImageOrientationDown:
        case UIImageOrientationDownMirrored:
            transform = CGAffineTransformTranslate(transform, self.size.width, self.size.height);
            transform = CGAffineTransformRotate(transform, M_PI);
            break;
        case UIImageOrientationLeft:
        case UIImageOrientationLeftMirrored:
            transform = CGAffineTransformTranslate(transform, self.size.width, 0);
            transform = CGAffineTransformRotate(transform, M_PI_2);
            break;
        case UIImageOrientationRight:
        case UIImageOrientationRightMirrored:
            transform = CGAffineTransformTranslate(transform, 0, self.size.height);
            transform = CGAffineTransformRotate(transform, -M_PI_2);
            break;
        default:
            break;
    }
    switch (self.imageOrientation) {
        case UIImageOrientationUpMirrored:
        case UIImageOrientationDownMirrored:
            transform = CGAffineTransformTranslate(transform, self.size.width, 0);
            transform = CGAffineTransformScale(transform, -1, 1);
            break;
        case UIImageOrientationLeftMirrored:
        case UIImageOrientationRightMirrored:
            transform = CGAffineTransformTranslate(transform, self.size.height, 0);
            transform = CGAffineTransformScale(transform, -1, 1);
            break;
        default:
            break;
    }
    CGContextRef ctx = CGBitmapContextCreate(NULL, self.size.width, self.size.height,
                                             CGImageGetBitsPerComponent(self.CGImage), 0,
                                             CGImageGetColorSpace(self.CGImage),
                                             CGImageGetBitmapInfo(self.CGImage));
    CGContextConcatCTM(ctx, transform);
    switch (self.imageOrientation) {
        case UIImageOrientationLeft:
        case UIImageOrientationLeftMirrored:
        case UIImageOrientationRight:
        case UIImageOrientationRightMirrored:
            CGContextDrawImage(ctx, CGRectMake(0,0,self.size.height,self.size.width), self.CGImage);
            break;
        default:
            CGContextDrawImage(ctx, CGRectMake(0,0,self.size.width,self.size.height), self.CGImage);
            break;
    }
    CGImageRef cgimg = CGBitmapContextCreateImage(ctx);
    UIImage *img = [UIImage imageWithCGImage:cgimg];
    CGContextRelease(ctx);
    CGImageRelease(cgimg);
    return img;
}
@end

修改转换Mat的代码为：

cv::Mat tempMat1 = [[largerImage fixOrientation] CVMat];
cv::Mat tempMat2 = [[subImage fixOrientation] CVMat];

3. 可选优化：统一转换为灰度图 #

放开代码中注释的转灰度逻辑，避免因为包内图和拍摄图通道数不一致（比如拍摄图是4通道RGBA，子图是3通道RGB）导致的其他匹配错误：

cv::cvtColor(tempMat1, tempMat1, cv::COLOR_RGB2GRAY);
cv::cvtColor(tempMat2, tempMat2, cv::COLOR_RGB2GRAY);

内容的提问来源于stack exchange，提问作者Anand