ImageMagick Perspective Distortion performance - opengl

Question

My requirement is perspective rotating a user submitted image into a pre-rendered background image (which is actually an image per frame on a video).
The easiest was to use ImageMagick and I wrote a very crude simple bash script to achieve what I needed as follows:
#!/bin/bash
# #author: neurosys
# #Description: Perspective transforms and projects an alpha image
# onto a background image.
if [ $# -ne 11 ]
then
echo 'Usage: ./map_image.sh background.jpg image.png output.jpg x1 y1 x2 y2 x3 y3 x4 y4';
exit;
fi
BG=$1
IMAGE=$2
DEST=$3
TEMP='temp.png'
BG_SIZE_W=$(convert $BG -print "%w\n" /dev/null)
BG_SIZE_H=$(convert $BG -print "%h\n" /dev/null)
IMAGE_W=$(convert $IMAGE -print "%w\n" /dev/null)
IMAGE_H=$(convert $IMAGE -print "%h\n" /dev/null)
X1=$4
Y1=$5
X2=$6
Y2=$7
X3=$8
Y3=$9
X4=${10}
Y4=${11}
OFFSET=15
TRANSFORM="$OFFSET,$OFFSET, $X1,$Y1 $(($IMAGE_W+$OFFSET)),$OFFSET $X2,$Y2 $OFFSET, $(($IMAGE_H+$OFFSET)) $X3,$Y3 $(($IMAGE_W+$OFFSET)), $(($IMAGE_H+$OFFSET)) $X4,$Y4"
echo "Transform matrix: $TRANSFORM"
convert $IMAGE -background transparent -extent $BG_SIZE_W\x$BG_SIZE_H-$OFFSET-$OFFSET $TEMP
convert $TEMP -background transparent -distort Perspective "$TRANSFORM" $TEMP
convert $BG $TEMP -composite $DEST
rm -f $TEMP
However, it takes for about 4 seconds to produce the desired image on my computer as follows:
[neuro#neuro-linux ~]$ time ./map_image.sh bg.png Hp-lovecraft.jpg output.jpg 494 108 579 120 494 183 576 196 && nomacs output.jpg
Transform matrix: 15,15, 494,108 195,15 579,120 15, 267 494,183 195, 267 576,196
real 0m3.852s
user 0m3.437s
sys 0m0.037s
[neuro#neuro-linux ~]$
The order of operations as well as the parameters I use in the above ImageMagick script might not be optimal. So, any opinions or alternatives to achieve what I need are greatly welcome.
The images used for the above example are,
Background
User submitted image
Output
I am wondering if there is a way to speed up this so much so that I can generate frames for a one minute long video (25 fps * 60 sec) under a few seconds?
As a matter of fact, in case this approach fails, I may resort to writing an OpenGL program for this specifically, which I believe will be much faster given hardware leveraging.
Somewhat off-topic note: The background image is prerendered in animation software (3ds Max). In case I resort to writing an opengl renderer, I can import mesh and camera from 3ds Max and do so for better perspective and lighting.
Thanks.
Edit:
With the help of guys over at ImageMagick forum, the bottleneck turned out in the first conversion of -extent, which was unneeded.
I ended up combining all the commands into one:
convert image.png -background transparent +distort Perspective "1,1, 494,108 201,1 579,120 1, 201 494,183 201, 201 576,196" -compose DstOver -composite bg.png out.png
It runs in 0.6 seconds but transparency somehow does not work, so the output image ends up being only the distorted image with black background all around.
Edit:
Someone on ImageMagick forums wrote a very fast and clean script that reduced it to 0.13 seconds.
Here is the link, in case anyone needs it:
https://www.imagemagick.org/discourse-server/viewtopic.php?f=1&t=29495&p=132182#p132141

Answer 1

Try using MPC format as your $TEMP instead of PNG. Encoding of MPC is much much faster. It's designed for use as a temporary file, for use with ImageMagick.
MPC actually creates two files, *.mpc and *.cache, so you need to
remove both. In your script, set TEMP=temp.mpc and TEMPCACHE=temp.cache,
and then at the end of the script, rm $TEMP TEMPCACHE
See the MPC entry on the ImageMagick Formats page.

Answer 2

If I get the dimensions of an image using your technique, it takes around 0.4 seconds for width and another 0.4 seconds for height. I mean like this:
BG_SIZE_W=$(convert $BG -print "%w\n" /dev/null) # 0.48 sec
BG_SIZE_H=$(convert $BG -print "%h\n" /dev/null) # 0.48 sec
If you get both the width and the height in one go like this, it takes 0.006 seconds altogether on my machine:
read BG_SIZE_W BG_SIZE_H < <(identify -ping -format "%w %h" bg.png)
I am still looking at the rest of your code.