iOS 拍摄多个屏幕截图 [英] iOS Take Multiple Screen Shots

问题描述

我有一个包含视频的 NSURL，我想每秒钟录制十次该视频的帧.我有代码可以捕捉我的播放器的图像，但我无法将其设置为每秒捕捉 10 帧.我正在尝试这样的事情，但它返回了相同的视频初始帧，正确的次数?这是我所拥有的:

I have an NSURL that contains a video, and I want to record a frame of that video ten times every seconds. And I have code that will capture an image of my player, but I have trouble setting it up to take capture 10 frames per second. I am trying something like this, but it is returning the same initial frame of the video, the correct number of times? Here is what I have:

AVAsset *asset = [AVAsset assetWithURL:videoUrl];
CMTime vidLength = asset.duration;
float seconds = CMTimeGetSeconds(vidLength);
int frameCount = 0;
for (float i = 0; i < seconds;) {
    AVAssetImageGenerator *imageGenerator = [[AVAssetImageGenerator alloc]initWithAsset:asset];
    CMTime time = CMTimeMake(i, 10);
    CGImageRef imageRef = [imageGenerator copyCGImageAtTime:time actualTime:NULL error:NULL];
    UIImage *thumbnail = [UIImage imageWithCGImage:imageRef];
                                CGImageRelease(imageRef);
NSString* filename = [NSString stringWithFormat:@"Documents/frame_%d.png", frameCount];
NSString* pngPath = [NSHomeDirectory() stringByAppendingPathComponent:filename];

[UIImagePNGRepresentation(thumbnail) writeToFile: pngPath atomically: YES];
frameCount++;
i = i + 0.1;
}

但不是获取视频当前时间 i 的帧，而是获取初始帧?

But instead of getting the frame at the current time i of the video, I just get the initial frame?

如何获得每秒 10 次视频的帧?

How can I get the frame of the video 10 times a second?

感谢您的帮助:)

推荐答案

您正在获取初始帧，因为您正在尝试借助浮点值创建 CMTime:

You are getting initial frame because you are trying to create CMTime with help of float value:

CMTime time = CMTimeMake(i, 10);

由于CMTimeMake函数将int64_t值作为第一个参数，你的float值将被四舍五入为int，你会得到不正确的结果.

Since CMTimeMake function takes int64_t value as first parameter, your float value will be rounded to int, and you will get incorrect result.

让我们稍微修改一下代码:

Lets change your code a bit:

1) 首先，您需要找到需要从视频中获取的总帧数.您写道，您需要每秒 10 帧，因此代码将是:

1) At first, you need to find total frames count that you need to get from the video. You wrote that you need 10 frames per second, so the code will be:

int requiredFramesCount = seconds * 10;

2) 接下来，您需要找到一个值，该值将在每一步中增加您的 CMTime 值:

2) Next you need to find a value that will be increasing your CMTime value on each step:

int64_t step = vidLength.value / requiredFramesCount;

3) 最后，您需要将 requestedTimeToleranceBefore 和 requestedTimeToleranceAfter 设置为 kCMTimeZero，以获取精确时间的帧:

3) And lastly, you need to set requestedTimeToleranceBefore and requestedTimeToleranceAfter to kCMTimeZero, to get a frame at precise time:

imageGenerator.requestedTimeToleranceAfter = kCMTimeZero;
imageGenerator.requestedTimeToleranceBefore = kCMTimeZero;

您的代码如下所示:

CMTime vidLength = asset.duration;
float seconds = CMTimeGetSeconds(vidLength);

int requiredFramesCount = seconds * 10;
int64_t step = vidLength.value / requiredFramesCount;

int value = 0;

for (int i = 0; i < requiredFramesCount; i++) {

    AVAssetImageGenerator *imageGenerator = [[AVAssetImageGenerator alloc]initWithAsset:asset];
    imageGenerator.requestedTimeToleranceAfter = kCMTimeZero;
    imageGenerator.requestedTimeToleranceBefore = kCMTimeZero;

    CMTime time = CMTimeMake(value, vidLength.timescale);

    CGImageRef imageRef = [imageGenerator copyCGImageAtTime:time actualTime:NULL error:NULL];
    UIImage *thumbnail = [UIImage imageWithCGImage:imageRef];
    CGImageRelease(imageRef);
    NSString* filename = [NSString stringWithFormat:@"Documents/frame_%d.png", i];
    NSString* pngPath = [NSHomeDirectory() stringByAppendingPathComponent:filename];

    [UIImagePNGRepresentation(thumbnail) writeToFile: pngPath atomically: YES];

    value += step;
}

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