Scaling error in [buildDefaultVideoComposition]

[gitgjogh]

Hi Olivier,

Nice work for video export on Apple system by offering customizable audio&video settings.

I recently made an alternative implementation by reference of your codes. And found that
there is some error in [buildDefaultVideoComposition] when you try to make original
video center inside the target size.

The problem is you scaling the video differently in x/y directions when the target DAR
(w/h ratio) is not same to the original video.

	matrix = CGAffineTransformScale(matrix, ratio / xratio, ratio / yratio);  

That means you havn't preserved the aspect ratio of the original video after decoding.

Someone may wondering why the exported video seems all right. That just becasue
AVVideoScalingModeResize is used implicitly for AVVideoScalingModeKey in the settings
of AVAssetWriterInput

	[AVAssetWriterInput assetWriterInputWithMediaType:AVMediaTypeVideo 
	                                   outputSettings:self.videoSettings];

As Apple comments:

	/* AVVideoScalingModeResize - Crop to remove edge processing region; 
	scale remainder to destination area.  Does not preserve aspect ratio. */

That means the exported video has been deformed twice during the video export. Deformation
is not gool for keeping video qualities, expecially when target DAR is reciprocal to the
original video DAR.

The fix is quite simple, just apply preferredTransform of the original videoTrack and
then set AVVideoScalingModeKey to AVVideoScalingModeResizeAspect. Here is my patch:

diff --git a/SDAVAssetExportSession.m b/SDAVAssetExportSession.m
index e99993b..86c7025 100755
--- a/SDAVAssetExportSession.m
+++ b/SDAVAssetExportSession.m
@@ -325,22 +325,12 @@
 		naturalSize.height = width;
 	}
 	videoComposition.renderSize = naturalSize;
-	// center inside
-	{
-		float ratio;
-		float xratio = targetSize.width / naturalSize.width;
-		float yratio = targetSize.height / naturalSize.height;
-		ratio = MIN(xratio, yratio);
-
-		float postWidth = naturalSize.width * ratio;
-		float postHeight = naturalSize.height * ratio;
-		float transx = (targetSize.width - postWidth) / 2;
-		float transy = (targetSize.height - postHeight) / 2;
-
-		CGAffineTransform matrix = CGAffineTransformMakeTranslation(transx / xratio, transy / yratio);
-		matrix = CGAffineTransformScale(matrix, ratio / xratio, ratio / yratio);
-		transform = CGAffineTransformConcat(transform, matrix);
-	}
+    // center inside
+    if (self.videoSettings && ![self.videoSettings objectForKey:AVVideoScalingModeKey]) {
+        NSMutableDictionary *internalSettings = [NSMutableDictionary dictionaryWithDictionary:self.videoSettings];
+        [internalSettings setObject:AVVideoScalingModeResizeAspect forKey:AVVideoScalingModeKey];
+        self.videoSettings = internalSettings;
+    }
 
 	// Make a "pass through video track" video composition.
 	AVMutableVideoCompositionInstruction *passThroughInstruction = [AVMutableVideoCompositionInstruction videoCompositionInstruction];

[KarenK1995]

Hi @gitgjogh. I opened an issue, can you look it? #90