How to convert a coordinates to indice array (vice versa) in a NetworkX graph?

Question

I want to use a raster for a A* and bidirectional Dijkstra path analysis in NetworkX. I am using Python for this project.

Raster example (it's a png file converted when uploaded, but the real problem is TIFF): Here

First I read in the raster with GDAL

input_raster = "raster.tif"
raster = gdal.Open(input_raster)

Next I read the raster as an array

bandraster = raster.GetRasterBand(1)
arr = bandraster.ReadAsArray()

So, I'll transform coords using a function:

def coord2pixelOffset(rasterfn, x, y):
    raster = gdal.Open(rasterfn)
    geotransform = raster.GetGeoTransform()
    originX = geotransform[0]
    originY = geotransform[3]
    pixelWidth = geotransform[1]
    pixelHeight = geotransform[5]
    xOffset = int((x - originX)/pixelWidth)
    yOffset = int((y - originY)/pixelHeight)
    return xOffset, yOffset

CostSurfacefn = 'raster.tif'
source_coord = (-41.1823753163, -13.83393276)
target_coord = (-40.3726182077, -14.2361991946)

# coordinates to array index
source = coord2pixelOffset(CostSurfacefn, source_coord[0], source_coord[1])
target = coord2pixelOffset(CostSurfacefn, target_coord[0], target_coord[1])

Next, the graph is loaded and function is called:

G = nx.DiGraph(arr)
length, path = nx.bidirectional_dijkstra(G, source, target)

But, the function return a error:

raise NetworkXError("The node %s is not in the digraph."%(n,))

networkx.exception.NetworkXError: The node (2612, -352) is not in the digraph.

How to do this in correct way?