Imagemagick processes hang forever on Loopbackjs - imagemagick

Question

I got this code to check if an image file contains blue pixels with Imagemagick and counting them - then saving the result.
It works well, but it seems like many processes of Imagemagick hang forever on the server and are making it very slow.
Is there a way to improve this code and avoid this trouble?
module.exports = function (File) {
File.observe('after save', function countPixels(ctx, next) {
if (ctx.instance && !ctx.instance.blue_pixels) {
var exec = require('child_process').exec;
// Convert file to retrieve only blue pixels:
exec('convert ' + ctx.instance.path + ' -fx "u.b>(u.g+0.2)&&u.b>(u.r+0.2)&&saturation>0.6" -format "%[fx:mean*w*h]" info:',
function (error, stdout, stderr) {
if (error !== null) {
return next(error);
} else {
ctx.instance.blue_pixels = stdout;
File.upsert(ctx.instance);
}
});
}
next();
});
};

Answer 1

The -fx operator that you are using is notoriously slow - especially for large images. I had a try at casting the same formula using faster methods which may help you. So, I made a sample image:
convert xc:red xc:lime -append \( xc:blue xc:cyan -append \) +append -resize 256x256! input.png
And then rewrote your expression like this:
convert input.png \
\( -clone 0 -separate -delete 0 -evaluate-sequence subtract -threshold 20% -write BG.png \) \
\( -clone 0 -separate -delete 1 -evaluate-sequence subtract -threshold 20% -write BR.png \) \
\( -clone 0 -colorspace hsl -separate -delete 0,2 -threshold 60% -write S.png \) \
-delete 0 \
-evaluate-sequence min result.png
Note that the -write XYZ.png are just debug statements that can be removed.
Basically, I am building a mask of all pixels that meet your criteria and making them white, and making all the ones that don't match your criteria black and at the end, I run -evaluate-sequence min to find the minimum of each pixel so that all three of your conditions must effectively be met:
that blue exceeds green by 20%
that blue exceeds red by 20%
that the saturation exceeds 60%
The -separate -delete N splits your image into RGB channels and then deletes one of the resulting channels, so if I -delete 1 (that is the Green channel) I am left with Red and Blue. Here are the intermediate, debug images. The first one is the condition Blue exceeds Red by 20%:
Then that Blue exceeds Green by 20%:
And finally that the Saturation exceeds 60%:
And then the result:
You'll need to put your -format "%[fx:mean*w*h]" info: back on the end in place of the output image name to get the count of saturated blue pixels.
If I run your command:
convert input.png -fx "u.b>(u.g+0.2)&&u.b>(u.r+0.2)&&saturation>0.6" result.png
My brain is not quite right today, so please run some checks - I may have something back-to-front somewhere!
As a benchmark, on a 10,000x10,000 pixel PNG, my code runs in 30 seconds, whereas the -fx equivalent takes nearly 7 minutes.

Answer 2

I don't know imagelagick part. But for node part I see that you call next non regarding to imagemgick opertion.
module.exports = function (File) {
File.observe('after save', function countPixels(ctx, next) {
if (ctx.instance && !ctx.instance.blue_pixels) {
var exec = require('child_process').exec;
// Convert file to retrieve only blue pixels:
exec('convert ' + ctx.instance.path + ' -fx "u.b>(u.g+0.2)&&u.b>(u.r+0.2)&&saturation>0.6" -format "%[fx:mean*w*h]" info:',
function (error, stdout, stderr) {
if (error !== null) {
return next(error);
} else {
ctx.instance.blue_pixels = stdout;
File.upsert(ctx.instance);
next();
}
});
}
else{next();}
//next(); //run next hook ASAP (before imagemagick returns back the result)
});
};