Best way to gather all pixels location (XY) inside a closed contour?

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Currently i have a black&white 3D model sliced in 2D images (+/- 500 to 1000 images), each layer is 0.05mm in Z, that aside i need to capture all closed black areas contours which i already have. I need to detect all empty spaces on the model looping all images and go up and down in Z at each countour area to find if is hollow space in a 3D prespective or if have any gap to outside the model and will not consider that area hollow.

Currently i'm doing the following:

1. for first layer to last layer:
2. Detect the required contours
3. Foreach contour
4. Create a empty image at same size of input
5. FillConvexPoly at empty image
6. Loop image pixels and find that previous draw pixels and save them in a array

That's the first part of the problem, what i'm doing is very inefficient/slow,... As can be observed each contour in layer image must loop all pixels in image, so if i have 5 countours on a image, it loop image 5 times to get pixels as a countour area.

Current code:

for (int i = 0; i < contours.Size; i++)
{
    if ((int)arr.GetValue(0, i, 2) != -1 || (int)arr.GetValue(0, i, 3) == -1) continue;

    grayscale.Dispose();
    grayscale = new Image<Gray, byte>(image.Width, image.Height);
    //grayscale = grayscale.CopyBlank();
    grayscale.FillConvexPoly(contours[i].ToArray(), new Gray(125));
    List<Point> points = new List<Point>();

    byte[,,] data = grayscale.Data;
    for (int y = 0; y < grayscale.Rows; y++)
    {
        for (int x = 0; x < grayscale.Cols; x++)
        {
            if(data[y, x, 0] != 125) continue;
            points.Add(new Point(x, y)); // Gather pixels
        }
    }
    listHollowArea.Add(new LayerHollowArea(points.ToArray()));
}

The question: Is there any function or code i can use to skip 4 to 6 and get all pixels inside the matched countours without that much effort?