LUT: Fix color definition and sampling.

[Mugen87]

Fixed #23549.

Description

Fixes getColor() and setColorMap(). Improves updateCanvas().

@dcriado @scurest Does this look better to you?

[scurest]

No, sorry. Maybe I wasn't clear, but rounding is the right thing to do. Here's an example:

Let's add a keyword 'blacktowhite': [[ 0.0, 0x000000 ], [ 1.0, 0xFFFFFF ]]. This is easy to understand since sampling at alpha ideally produces r=g=b=alpha.

With this PR's Lut:

>>> const l = new Lut("blacktowhite", 4);

// this returns lut[1] (0.25), it should return lut[2] (0.5)
// since 0.48 is closer to 0.5 than 0.25
>>> l.getColor(0.48)
Object { r: 0.25, g: 0.25, b: 0.25 }

// behavior for alpha near 1
>>> l.getColor(0.999)
Object { r: 0.75, g: 0.75, b: 0.75 }  // bad
>>> l.getColor(1)
Object { r: 1, g: 1, b: 1 }  // good!

With dev's Lut:

// this is correct
>>> l.getColor(0.48)
Object { r: 0.5, g: 0.5, b: 0.5 }

// the only "problem" is for alpha near 1 the color at
// the very end of the ramp doesn't get returned
// (essentially the lut[n] bucket is missing/unused)
>>> l.getColor(0.999)
Object { r: 0.75, g: 0.75, b: 0.75 }
>>> l.getColor(1)
Object { r: 0.75, g: 0.75, b: 0.75 }

// contrast with alpha near 0, where the color at
// the beginning DOES get returned
>>> l.getColor(0.001)
Object { r: 0, g: 0, b: 0 }

[Mugen87]

Okay, I've updated the PR.

[scurest]

Cool, getColor looks right to me.

For setColorMap:

The loop doesn't always make n+1 elements in lut. For n=11 for example, i ends up being > 1 at the end of the loop, so there is no lut[11] and getColor(1) returns undefined.

IMO it would be easier to do something like this

// sample at 0
this.lut.push(this.map[0][1])

// sample at 1/n, ..., (n-1)/n
let j = 0;
for (i = 1; i !== n; i++) {  // use integer loop counter
    const alpha = i * step;

    // advance j until alpha is in the right range
    ...

    const color = lerp(...)

    // every iteration unconditionally pushes one element
    this.lut.push(color)
}

// sample at 1
this.lut.push(this.map[this.map.length - 1][1])

[Mugen87]

Slightly changed your code listing but the result should be okay now.

[scurest]

Okay. As far as I can tell, it's right now.

[mrdoob]

Thanks!

[dcriado]

Sorry guys, I could not have a look at this until now. It really looks good now! Thank you so much @Mugen87!!