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--- doc:appunti:linux:video:fix_smartphone_portrait_videos [2018/04/07 07:48] – niccolo
+++ doc:appunti:linux:video:fix_smartphone_portrait_videos [2018/04/07 15:47] (current) – [The Gradient Overlay Mask] niccolo
@@ Line 8: / Line 8: @@
 {{original_video_black_bands.jpg?direct&300|The video viewd with lateral black bands}}
-{{final_video_interpolated.jpg?direct&300|The interpolated video}}
+===== The Compromise Solution =====
+A compromise solution is to **cut out a central area of the video** and **add to the sides some artifact** that simulates the presence of a portion of video that does not actually exist. I find that solution by far more acceptable than the original video:
+{{final_video_interpolated.jpg?direct&380|The interpolated video}}
+So we have to:
+  - Choose a **region to crop** from the original video. It can vary from a square to the full portrait rectangle.
+  - Make a **fake background** blurring the cropped region and stretching it to the 16:9 aspect ratio.
+  - **Overlay** the cropped region over the fake background.
+{{choose_crop_area.png?direct&280 |Choose the crop area}}
+We wrote a **[[#the_python_script|Python script]]** to assist in step #1, which calculate all the required numbers just provinding two parameters: the **crop_aspect** and the **offset_y**.
+Adjust the **crop_aspect** from 0.0 (which selects a square) to 1.0 (which selects the full rectangle of the picture). Using a square-shaped crop area we will get the **highest zoom** level into the video, but we will cut-out  a greather part of the frames, above and below the cropped region.
+We can move the cropped region to the top of the image by setting **offset_y** to 0.0. Move it to te bottom by setting it to 1.0. To crop the image at the **center** of the screen, use an //offset_y// of **0.5**. If you use a //crop_aspect// equal to 1.0 (full frame rectangle), the //offset_y// will be ignored.
+===== Cropped Region and Fake Background =====
+Once selected the cropped region, a **fake background** is created by simply **stretching** the region to **16:9 aspect ratio** and applying a **blur effect**.
+{{cropped_region.jpg?direct&200|The Cropped Region}}
+{{fake_background.jpg?direct&356|The Fake Background}}
+===== The Gradient Overlay Mask =====
+When the cropped image is placed above the background, it is quite annoying to see the **sharp edge** of the image. A better approach is to gradually fade the image at the right and at left edges. This requires to blend the image and the background using a **gradient mask**. We prepared the mask as a **{{gradient_overlay_mask.png?linkonly|PNG image}}**, using **convert** from the ImageMagick suite.
+<code>
+convert -size 768x768 \
+    -define "gradient:bounding-box=768x38+0+0" -define "gradient:vector=0,37,0,0" \
+    gradient:none-black \
+    -define "gradient:bounding-box=768x768+0+730" -define "gradient:vector=0,730,0,767"
+    gradient:none-black \
+    -composite -channel a -negate -rotate 90 tmp_mask.png
+</code>
+**NOTICE**: It seems that [[https://www.imagemagick.org/discourse-server/viewtopic.php?f=3&t=33804|there is a bug]] in ImageMagick **gradient:bounding-box**, so we had to generate the mask in top-down mode and rotate it 90 degrees afterward.
+===== The ffmpeg Recipe =====
+The first step is to **remove the rotation metatag** from the input video. It seems that ffmpeg is unable to remove the metatag and apply the required video filters in the same pass, so we need to make a temporary copy of the input video file, without re-encoding it:
+<code>
+ffmpeg -i input_video.mp4 -c copy -metadata:s:v:0 rotate=0 tmp_norotate.mp4
+</code>
+finally we invoke **ffmpeg** to do all the magic. We use a **filter_complex** incantation:
+<code>
+ffmpeg -i tmp_norotate.mp4 -loop 1 -i tmp_mask.png -filter_complex "\
+  [0:v]split [a][b]; \
+  [a]transpose=1,crop=720:720:0:280,scale=768:768,setdar=768/768 [crop]; \
+  [b]transpose=1,crop=720:720:0:280,scale=1366:768,setdar=16/9,avgblur=54 [back]; \
+  [1:v]alphaextract [mask]; \
+  [crop][mask]alphamerge [masked]; \
+  [back][masked]overlay=299:0 \
+" video_out.mp4
+</code>
+The first video stream **%%[0:v]%%** coming from the video file is splitted in two. The first copy, called **%%[a]%%**, is used to crop the interesting part after the rotation (//transpose//), thus creating the **%%[crop]%%** stream. The second copy, called **%%[b]%%**, is used to create the background applying rotation, stretch and blur filters, thus creating the **%%[back]%%** stream. The mask image is used in loop to create another video stream **%%[1:v]%%**, from which we extract the alpha channel (transparency) creating a stream called **%%[mask]%%**. The **%%[crop]%%** stream is merged with the **%%[mask]%%**, providing the **%%[masked]%%** stream. Finally the **%%[masked]%%** stream is overlayed to the **%%[back]%%** one, to realize the final output.
+===== The Python Script =====
+The Python script **{{smartphonevideo_2landscape.txt|smartphonevideo_2landscape}}** will make the calculation to prepare the whole incantation: it will print on the standard output a **shell script** with the proper invokation of **convert** and **ffmpeg**. Use it in this way:
+<code>
+smartphonevideo_2landscape video_file [crop_aspect] [offset_y] > ffmpeg_recipe
+source ffmpeg_recipe
+</code>
+You have to edit the script to adjust the size of the input video (default 1280x720), the rotation needed (default 1 = 90 deg clockwise), and the size of output video produced (default 1366x768).