IOS使用AVFoundation在視頻上添加字幕以及控制字幕時間

IOS在視頻上添加字幕效果的基本思路是：

1. 使用自定義的CATextLayer文字圖層或者CAShapeLayer文字圖層，添加到視頻的Layer上創建用戶自定義的字幕效果。這兩者的區別是：CATextLayer支持設置簡單的文字效果，包括文字的內容、字體、字號大小、對其方式、文字顏色、背景顏色等基本的屬性；CAShapeLayer功能更強大，提供了CATextLayer沒有的邊框大小、邊框顏色等設置，如果需要更高級的文字內容展示，需要使用CATextLayer配合UIBezierPath來定製自定義的文字內容。

2. 通過設置Layer圖層的動畫來控制字幕的時間點和時間長度，這裏有一個坑如果單獨設置CATextLayer或者CAShapeLayer的動畫不能控制開始的時間，需要額外添加一個CALayer圖層，把文字圖層CATextLayer或者CAShapeLayer添加到父CALayer圖層中，在文字圖層CATextLayer或者CAShapeLayer上設置開始的動畫

    NSTimeInterval animatedInStartTime = startTime + initAnimationDuration;
    CABasicAnimation *fadeInAnimation = [CABasicAnimation animationWithKeyPath:@"opacity"];
    fadeInAnimation.fromValue = @0.0f;
    fadeInAnimation.toValue = @1.0f;
    fadeInAnimation.additive = NO;
    fadeInAnimation.removedOnCompletion = NO;
    fadeInAnimation.beginTime = animatedInStartTime;
    fadeInAnimation.duration = animationDuration;
    fadeInAnimation.autoreverses = NO;
    fadeInAnimation.fillMode = kCAFillModeBoth;
    [textLayer addAnimation:fadeInAnimation forKey:@"opacity"];

在父CALayer圖層上設置結束的動畫，這樣設置才能實現用戶自定義的時間點和時間長度

NSTimeInterval animatedOutStartTime = startTime + duration - animationDuration;
    CABasicAnimation *fadeOutAnimation = [CABasicAnimation animationWithKeyPath:@"opacity"];
    fadeOutAnimation.fromValue = @1.0f;
    fadeOutAnimation.toValue = @0.0f;
    fadeOutAnimation.additive = NO;
    fadeOutAnimation.removedOnCompletion = NO;
    fadeOutAnimation.beginTime = animatedOutStartTime;
    fadeOutAnimation.duration = animationDuration;
    fadeOutAnimation.autoreverses = NO;
    fadeOutAnimation.fillMode = kCAFillModeBoth;
    [animatedTitleLayer addAnimation:fadeOutAnimation forKey:@"opacity"];

完整的代碼

- (CALayer *)buildLayerbuildTxt:(NSString*)text
                       textSize:(CGFloat)textSize
                      textColor:(UIColor*)textColor
                    strokeColor:(UIColor*)strokeColor
                        opacity:(CGFloat)opacity
                       textRect:(CGRect)textRect
                       fontPath:(NSString*)fontPath
                     viewBounds:(CGSize)viewBounds
                      startTime:(NSTimeInterval)startTime
                       duration:(NSTimeInterval)duration
{
    if (!text || [text isEqualToString:@""])
    {
        return nil;
    }

    // Create a layer for the overall title animation.
    CALayer *animatedTitleLayer = [CALayer layer];

    // 1. Create a layer for the text of the title.
    CATextLayer *titleLayer = [CATextLayer layer];
    titleLayer.string = text;
    titleLayer.font = (__bridge CFTypeRef)(@"Helvetica");
    titleLayer.fontSize = textSize;
    titleLayer.alignmentMode = kCAAlignmentCenter;
    titleLayer.bounds = CGRectMake(0, 0, textRect.size.width, textRect.size.height);
    titleLayer.foregroundColor = textColor.CGColor;
    titleLayer.backgroundColor = [UIColor clearColor].CGColor;
    // [animatedTitleLayer addSublayer:titleLayer];

    // 添加文字以及邊框效果
    UIFont *font = nil;
    if ((fontPath != nil) && (fontPath.length > 0)) {
        font = [[FLVideoEditFontManager sharedFLVideoEditFontManager] fontWithPath:fontPath size:textSize];
        titleLayer.font = CGFontCreateWithFontName((__bridge CFStringRef)font.fontName);
    }
    if (font == nil) {
        titleLayer.font = (__bridge CFTypeRef)(@"Helvetica");
    }

    UIBezierPath *path = nil;
    if (font) {
        path = [FLLayerBuilderTool createPathForText:text fontHeight:textSize fontName:(__bridge CFStringRef)(font.fontName)];
    }
    else
    {
        path = [FLLayerBuilderTool createPathForText:text fontHeight:textSize fontName:CFSTR("Helvetica")];
    }
    CGRect rectPath = CGPathGetBoundingBox(path.CGPath);
    CAShapeLayer *textLayer = [CAShapeLayer layer];
    textLayer.path = path.CGPath;
    textLayer.lineWidth = 1;
    if (strokeColor != nil) {
        textLayer.strokeColor = strokeColor.CGColor;
    }
    if (textColor != nil) {
        textLayer.fillColor = textColor.CGColor;
    }
    textLayer.lineJoin = kCALineJoinRound;
    textLayer.lineCap = kCALineCapRound;
    textLayer.geometryFlipped = NO;
    textLayer.opacity = opacity;
    textLayer.bounds = CGRectMake(0, 0, rectPath.size.width, textSize+10);
    [animatedTitleLayer addSublayer:textLayer];

    // 動畫圖層位置
    animatedTitleLayer.position = CGPointMake(textRect.origin.x+textRect.size.width/2, viewBounds.height - textRect.size.height/2 - textRect.origin.y);

    NSTimeInterval initAnimationDuration = 0.1f;
    NSTimeInterval animationDuration = 0.1f;

    // 3.顯示動畫
    NSTimeInterval animatedInStartTime = startTime + initAnimationDuration;
    CABasicAnimation *fadeInAnimation = [CABasicAnimation animationWithKeyPath:@"opacity"];
    fadeInAnimation.fromValue = @0.0f;
    fadeInAnimation.toValue = @1.0f;
    fadeInAnimation.additive = NO;
    fadeInAnimation.removedOnCompletion = NO;
    fadeInAnimation.beginTime = animatedInStartTime;
    fadeInAnimation.duration = animationDuration;
    fadeInAnimation.autoreverses = NO;
    fadeInAnimation.fillMode = kCAFillModeBoth;
    [textLayer addAnimation:fadeInAnimation forKey:@"opacity"];

    NSTimeInterval animatedOutStartTime = startTime + duration - animationDuration;
    CABasicAnimation *fadeOutAnimation = [CABasicAnimation animationWithKeyPath:@"opacity"];
    fadeOutAnimation.fromValue = @1.0f;
    fadeOutAnimation.toValue = @0.0f;
    fadeOutAnimation.additive = NO;
    fadeOutAnimation.removedOnCompletion = NO;
    fadeOutAnimation.beginTime = animatedOutStartTime;
    fadeOutAnimation.duration = animationDuration;
    fadeOutAnimation.autoreverses = NO;
    fadeOutAnimation.fillMode = kCAFillModeBoth;

    [animatedTitleLayer addAnimation:fadeOutAnimation forKey:@"opacity"];

    return animatedTitleLayer;
}

依賴的工具類FLLayerBuilderTool.m文件：

#import "FLLayerBuilderTool.h"
#import <CoreText/CoreText.h>

@implementation FLLayerBuilderTool


+ (UIBezierPath*) createPathForText:(NSString*)string fontHeight:(CGFloat)height fontName:(CFStringRef)fontName
{
    if ([string length] < 1)
        return nil;

    UIBezierPath *combinedGlyphsPath = nil;
    CGMutablePathRef letters = CGPathCreateMutable();

    CTFontRef font = CTFontCreateWithName(fontName, height, NULL);
    if (font == nil) {
        font = (__bridge CFTypeRef)(@"Helvetica");
    }
    NSDictionary *attrs = [NSDictionary dictionaryWithObjectsAndKeys:
                           (__bridge id)font, kCTFontAttributeName,
                           nil];
    NSAttributedString *attrString = [[NSAttributedString alloc] initWithString:string
                                                                     attributes:attrs];
    CTLineRef line = CTLineCreateWithAttributedString((CFAttributedStringRef)attrString);
    CFArrayRef runArray = CTLineGetGlyphRuns(line);

    // for each RUN
    for (CFIndex runIndex = 0; runIndex < CFArrayGetCount(runArray); runIndex++)
    {
        // Get FONT for this run
        CTRunRef run = (CTRunRef)CFArrayGetValueAtIndex(runArray, runIndex);
        CTFontRef runFont = CFDictionaryGetValue(CTRunGetAttributes(run), kCTFontAttributeName);

        // for each GLYPH in run
        for (CFIndex runGlyphIndex = 0; runGlyphIndex < CTRunGetGlyphCount(run); runGlyphIndex++)
        {
            // get Glyph & Glyph-data
            CFRange thisGlyphRange = CFRangeMake(runGlyphIndex, 1);
            CGGlyph glyph;
            CGPoint position;
            CTRunGetGlyphs(run, thisGlyphRange, &glyph);
            CTRunGetPositions(run, thisGlyphRange, &position);

            // Get PATH of outline
            {
                CGPathRef letter = CTFontCreatePathForGlyph(runFont, glyph, NULL);
                CGAffineTransform t = CGAffineTransformMakeTranslation(position.x, position.y);
                CGPathAddPath(letters, &t, letter);
                CGPathRelease(letter);
            }
        }
    }
    CFRelease(line);

    combinedGlyphsPath = [UIBezierPath bezierPath];
    [combinedGlyphsPath moveToPoint:CGPointZero];
    [combinedGlyphsPath appendPath:[UIBezierPath bezierPathWithCGPath:letters]];

    CGPathRelease(letters);
    CFRelease(font);

    if (attrString)
    {
        attrString = nil;
    }

    return combinedGlyphsPath;
}

@end

參考資料：
視頻特效製作：如何給視頻添加邊框、水印、動畫以及3D效果
視頻特效製作2
AVFoundation Tutorial: Adding Overlays and Animations to Videos