Final version

map.js

const bounds = poster.bounds

// Use geoViewport to determine the center and the zoom level of the entire map.
const viewport = geoViewport.viewport([
  bounds['_sw'].lng,
  bounds['_sw'].lat,
  bounds['_ne'].lng,
  bounds['_ne'].lat
], [size.width, size.height], undefined, undefined, 512, true)

const zoom = viewport.zoom

const merc = new SphericalMercator({ size: 512 })

// Determine the x and y of each tiles of the matrix using the
// SphericalMercator library
const xyz = merc.xyz([
  bounds['_sw'].lng,
  bounds['_sw'].lat,
  bounds['_ne'].lng,
  bounds['_ne'].lat,
], zoom)

const coord = [xyz.minX, xyz.minY]
const nbXTiles = xyz.maxX - xyz.minX + 1
const nbYTiles = xyz.maxY - xyz.minY + 1
const images = []

for (let i=0; i<(nbXTiles*nbYTiles); i++) {
  // For each tile, find the bbox according to the x and y coords.
  const bbox = merc.bbox(coord[0], coord[1], zoom)
   
  // Find the center en the zoom level according to the bbox
  const imageViewport = geoViewport.viewport([
      bbox[0],
      bbox[1],
      bbox[2],
      bbox[3]
    ], [512, 512], undefined, undefined, 512, true)
  images.push(imageViewport)

  coord[1] = coord[0] === xyz.maxX ? coord[1] + 1 : coord[1]
  coord[0] = coord[0] === xyz.maxX ? xyz.minX : coord[0] + 1
}

// I'm now able to build the call to the Static Image API using more accurate centers and zoom levels.