Incorrect output for images around 300x300 pixels and up.

[sbj42]

node-pnglib seems to have some trouble producing images around 300x300 pixels and higher. The image seems to stop part-way. That is, the top part of the image looks right, but the bottom is broken in some way (may render as the background color, or may be "blank").

As the image size goes up, the area of the image that looks right stays the same, so I suspect some kind of limit of addressing or file size or something like that.

See the sample script below:

• node v8.11.1
• node-pnglib v1.5.5

const pnglib = require('node-pnglib');
const fs = require('fs');

const W = 300;
const H = 300;

const png = new pnglib(W, H, undefined, 'red');

for (let y = 0; y < H; y ++) {
    for (let x = 0; x < W; x ++) {
        png.setPixel(x, y, 'blue');
    }
}

fs.writeFileSync('test.png', png.getBuffer());

Expected output: a blue square.

[sbj42]

This code at pnglib.js:83 looks suspicious:

  // compute the index into a png for a given pixel
  index(x, y) {
    let i = y * (this.width + 1) + x + 1;
    let offset = this.idat_offs + 8 + 2 + 5;
    return offset * ((i / 0xffff) | 0 + 1) + i;
  }

I don't know what the problem is, but the pixel indexes around where I start seeing problems are about when i would be greater than 0xffff (e.g. approximately 256x256 pixels into the image).

[Curnull]

This code at pnglib.js:83 looks suspicious:

  // compute the index into a png for a given pixel
  index(x, y) {
    let i = y * (this.width + 1) + x + 1;
    let offset = this.idat_offs + 8 + 2 + 5;
    return offset * ((i / 0xffff) | 0 + 1) + i;
  }

I don't know what the problem is, but the pixel indexes around where I start seeing problems are about when i would be greater than 0xffff (e.g. approximately 256x256 pixels into the image).

Code should be like this:

index(x, y) {
let i = y * (this.width + 1) + x + 1;
return this.idat_offs + 8 + 2 + 5 * Math.floor(i / 0xffff + 1) + i
}

works for me!