Is it possible to stretch/scale only the center part of an overlay image?
I don't mean the middle section of a video, I mean only the center part of
a single image (png) that will be overlayed onto a background, such that
the left and right sides are relatively unchanged, but the center of the
image is stretched to fit the new size.
To further explain... I have a background image with a bunch of text lines
of varying length. I want to overlay an oval (actually a rounded
rectangle) over some text as a highlight. Since all the text parts are
different lengths, I'll either need to create a separate overlay image for
each one OR create one standard overlay image and "stretch/squeeze" it to
fit each section of text. I'd prefer to create only a single overlay
image.
The problem is, when I try to stretch/squeeze my rounded rectangle (using
scale) the ends either get stretched too much or squeezed too much
depending if I need to scale up or down. What I'd like to see is the ends
look the same (or close to) the original overlay image and only the center
part is stretched/squeezed.
Is there some filter I can use instead of or in addition to "scale" that
will do something like this?
Below is an example that illustrates the issue using the ImageMagick
"convert" command to generate the background and highlight images and
ffmpeg to overlay the highlight on the background.
What you'll notice is the highlight around the shortest text has the ends
squeezed too much and the highlight around the longest text has the ends
stretched too much.
Create a background image with 4 text lines:
$ convert -size 720x480 xc:black -font DejaVu-Sans -pointsize 20 -fill white -draw "text 200,100 'not scaled at
all'" -draw "text 200,200 'short'" -draw "text 200,300 'middle size line'" -draw "text
200,400 'very very long line with extra words" background.png
Create a rounded rectangle highlight for the overlay:
$ convert -size 200x50 xc:none -fill transparent -stroke green -strokewidth 5 -draw
"roundrectangle 10,5 190,45 20,20" highlight.png
Overlay the highlight on each text line and "stretch/squeeze" (using
scale) to fit the text width:
$ ffmpeg -loop 1 -i background.png -loop 1 -i highlight.png -codec:v
mpeg2video -b:v 16384k -bufsize 4096k -maxrate 30000k -filter_complex
'[1:0]scale=70:in_h[o1];[1:0]scale=190:in_h[o2];[1:0]scale=430:in_h[o3];[0:0][1:0]overlay=x=180:y=70[v1];[v1][o1]overlay=x=190:y=170[v2];[v2][o2]overlay=x=182:y=270[v3];[v3][o3]overlay=x=166:y=370'
-t 5 -f vob example.mpg
ffmpeg version N-96725-g13dc90396d Copyright (c) 2000-2020 the FFmpeg
developers
built with gcc 9 (GCC)
configuration:
libavutil 56. 40.100 / 56. 40.100
libavcodec 58. 68.102 / 58. 68.102
libavformat 58. 38.100 / 58. 38.100
libavdevice 58. 9.103 / 58. 9.103
libavfilter 7. 75.100 / 7. 75.100
libswscale 5. 6.100 / 5. 6.100
libswresample 3. 6.100 / 3. 6.100
Input #0, png_pipe, from 'background.png':
Duration: N/A, bitrate: N/A
Stream #0:0: Video: png, gray(pc), 720x480, 25 fps, 25 tbr, 25 tbn, 25 tbc
Input #1, png_pipe, from 'highlight.png':
Duration: N/A, bitrate: N/A
Stream #1:0: Video: png, pal8(pc), 200x50, 25 fps, 25 tbr, 25 tbn, 25 tbc
Stream mapping:
Stream #0:0 (png) -> overlay:main
Stream #1:0 (png) -> scale
Stream #1:0 (png) -> scale
Stream #1:0 (png) -> scale
Stream #1:0 (png) -> overlay:overlay
overlay -> Stream #0:0 (mpeg2video)
Press [q] to stop, [?] for help
Output #0, vob, to 'example.mpg':
Metadata:
encoder : Lavf58.38.100
Stream #0:0: Video: mpeg2video (Main), yuv420p, 720x480, q=2-31, 16384
kb/s, 25 fps, 90k tbn, 25 tbc (default)
Metadata:
encoder : Lavc58.68.102 mpeg2video
Side data:
cpb: bitrate max/min/avg: 30000000/0/16384000 buffer size: 4096000
vbv_delay: N/A
frame= 125 fps=0.0 q=2.0 Lsize= 378kB time=00:00:04.92 bitrate=
629.4kbits/s speed=11.9x
video:373kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB
muxing overhead: 1.427849%
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