如何让CALayer清楚？

Question

我有这个代码返回CATextLayer放置在视频上，它返回模糊文本。我在网上搜索过，大多数人建议添加textLayer.contentsScale，但这对我没用。

- (CATextLayer *)buildTextLayerWithText:(NSString *)text atPosition:(WatermarkPosition)position forVideoSize:(CGSize)videoSize
{
    CATextLayer *textLayer = [[CATextLayer alloc] init];

    NSMutableDictionary *attributes = [[NSMutableDictionary alloc] init];
    CGFloat fontSize = videoSize.height / 20.0;
    [attributes setObject:(__bridge id)(__bridge CFTypeRef)([ChannelsInterface mediumFontOfSize:fontSize]) forKey:(NSString*)kCTFontAttributeName];
    [attributes setObject:(__bridge id)[[UIColor whiteColor] CGColor] forKey:(NSString*)kCTForegroundColorAttributeName];
    [attributes setObject:[NSNumber numberWithFloat:fontSize] forKey:(NSString*)kCTFontSizeAttribute];;
    NSAttributedString *attributedString = [[NSAttributedString alloc] initWithString:text attributes:attributes];

    textLayer.string = attributedString;
    CGSize textLayerSize = [text sizeWithAttributes:attributes];
    textLayer.contentsScale = [UIScreen mainScreen].scale;
    [textLayer setFrame:getFrameForPosition(textLayerSize, videoSize, position)];
    textLayer.shadowColor = [UIColor blackColor].CGColor;
    textLayer.shadowRadius = 1.0f;
    textLayer.shadowOpacity = 0.5f;
    textLayer.shadowOffset = CGSizeMake(0.0f, 0.0f);
    [textLayer setRasterizationScale:[UIScreen mainScreen].scale];
    [textLayer setShouldRasterize:YES];

    return textLayer;
}