Issue while calculating float value using NodeJS N-API

Question

Hello everyone,

Coming from the web dev world. I am currently trying to do some C code which is converting an RGB value to an XYZ value that can be used by NodeJS through N-API. The issue that I have is regarding to float calculation. Below is the explanation of my issue:

Based on this C code below, this code is trying to convert an RGB value to an XYZ value.

char * colorType = getStringValue(env, funcParams[2], SPACELen);
// m is either the value srgb | adobeRgb
Matrix m = getEnumFromStr(colorType);
Rgb * rgb = getRGBFromJSObj(env, funcParams[0]);
xyz = generateXyzFromRgb(rgb, m);

And I am using this JS snippet to call my library

const rgb = {
  r: 255,
  g: 255,
  b: 255
};

const xyz = lib.getXyzFromRgb(rgb, "srgb", 10000);
expect(xyz).to.be.deep.equal({
  x: 0.9504,
  y: 1,
  z: 1.0888
});

If everything should be all right the output should be like the one below

{
 x: 0.9504,
 y: 1,
 z: 1.0888 
}

However the output that I have is this one

{
 x: 0.9502,
 y: 0.9997,
 z: 1.0886 
}

As you can see the output is totally wrong. However this wrong output only happened on my local machine (OSX) and only when I am trying to do the conversion by using the JS snippet.

Indeed, when I am trying to run the conversion with this piece of code below directly through Xcode the output is correct

// RGB and & m variable is outputing the same value as the conversion done by the C code above
xyz = generateXyzFromRgb(rgb, m);

Moreover when I'm trying to call the JS code through travis which also running OSX and on Ubuntu through Docker the JS code also output the right value.

Can it be more related to the hardware or the way I am compiling my libraries or else ?

Thank you in advance.

Also in general in any programming language where numbers are binary floating-point values, performing strict equality tests is tricky. — Pointy, Nov 20 '18 at 22:47
@Pointy the numeric data type is float. Should i change this to double ?. Indeed I looked at the IEE754 which looks to be tricky... However I find it strange that the wrong output only happened on my machine and only when using the JS snippet and not on travis osx or a in ubuntu — Yellowman, Nov 20 '18 at 23:01
The C `float` type is (usually) only 32 bits; JavaScript numbers are 64 bits. — Pointy, Nov 20 '18 at 23:11
@Pointy Hmm yes, behind the usage of the float value I am converting the float to the double value in order for the value to be used by N-API. Do you think that there might be a loss of precision during this conversion ? I guess I should try to convert it right away into double and see how it goes — Yellowman, Nov 20 '18 at 23:19

score 2 · Accepted Answer · answered Nov 21 '18 at 00:49

In javascript v8 engine. Actually only exist smi and double. readFloatBE: when float pass to v8, the float will be cast to double. it will promote to the double when you go out and rounding into a float.

First, if you really want to get value like C, you need to manually specify the rounded digits and re-instantiate the Number object with the Number.prototype.toPrecision function rounded up:

For your reference:

var v = 5.2
var buffer = new Buffer(5)
buffer.writeFloatBE(v)
var g = buffer.readFloatBE()

console.log(v)
console.log(g)
console.log(v==g)
console.log(Number(g.toPrecision(5)))

Hmm indeed. I could return a buffer instead of a float value that might help in term of precision — Yellowman, Nov 21 '18 at 08:26

score 0 · Answer 2 · answered Nov 21 '18 at 21:35

Indeed, as post by Pointy and user10683038 JavaScript numbers are 64 bits.

As I coded this feature the numeric type used was float (was not thinking at making a Node module at first glance). Later on I decided to do a conversion from float to double using a small util method like below

double v = *(double *) arg;
status = napi_create_double(env, v, &value);

Strangely it appear that I might have randomly loosing decimal precision while doing this conversion while it should have not (32bit to 64bit) or I might have missing out something.

Later on I refactor the same code but this time by using double instead and it works as it should. Many thanks to all of you.

Note: As I used this util code above method accross the NodeJS binding codebase I am surprised that I haven't got a decimal precision error earlier.

Issue while calculating float value using NodeJS N-API

2 Answers2