Calculates the luminance value and histogram of JPEG images. This is based on Bill Yeh's excellent node-pixelr (https://github.com/billyeh/node-pixelr), adapted for getting luminance and histogram data instead of raw pixels.
I was originally using node-pixelr and calculating luminance and histogram data in javascript, but I thought it would be faster to do it in C++ before sending it to js. In the end, I can't say I notice much difference in speed, though I haven't done any benchmarking.
The method for luminance is something I came up with after some trial and error. I don't know if it's correct or ideal, but it's working for my application (adjusting exposure based on the luminance).
0.2.1 Update: build error fix 0.2.0 Update: changed algorithm to be lookup table based instead of trying to calculate, removed histogram array for now 0.1.0 Update: added "clipped" property to results; percentage (0-1) of pixels at 255 0.0.4 Update: further refined luminance value to match camera exposure 0.0.3 Update: changed gamma correction to 1/2.2 0.0.2 Update: converted pixel value to linear (removed gamma correction) before calculating luminance
var luminance = require('jpeg-lum');
/** luminance.read(filename, callback)
* Filename points to a JPEG image
* The callback takes an object with properties 'histogram' (an array), 'luminance', 'clipped' (0-1, percentage of pixels at 255), width', and 'height'.
*/
luminance.read("image.jpeg", function(err, res) {
console.log("Luminance:", res.luminance);
console.log("Percentage clipped:", res.clipped);
console.log("Histogram Array:", res.histogram);
});
JPEG parsing uses libjpeg
$ npm install jpeg-lum
(The MIT License)
Modifications for jpeg-lum Copyright (c) 2016 Elijah Parker
Pixelr codebase Copyright (c) 2013 Bill Yeh
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The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
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