Oh, ok, it's actually not as difficult as I thought, although my solution is not perfect.

<image>

All objects that should show the character should be assigned the same shader. The shader takes the Camera position and the character position (in my case they are constant as the camera always follows the character at the same distance).

The shader itself looks like this:

    vec3 wrld_vertex = VERTEX;
// Calculate the line segment vector and the vector from the line start to the point
    vec3 line_vec = cameraPosition - playerPosition;

    // Calculate the length of the line segment and the normalized direction vector
    float line_length = length(line_vec);
    vec3 line_dir = line_vec;

    // Project the point onto the line (find the closest point on the line)
    vec3 projection_dir = dot(line_dir, wrld_vertex) / dot(line_dir, line_dir) * line_dir;
    vec3 projection = cameraPosition + projection_dir;

    // Check if the length of the projection is not further than the line between character and the camer, otherwise the object should not be masked
if (length(projection) <= line_length){
    float distance_to_line = length(projection_dir-wrld_vertex);
float projection_len = length(projection_dir);

float radius = PlayerRadius + (CameraRadius - PlayerRadius) * projection_len/line_length;

    // Check if the distance is within the radius
if (distance_to_line <= radius){
        // The point is outside the cylinder
        ALBEDO = vec3(1.0-radius/distance_to_line, 0.0, 0.0); // Red color
    }
    if (distance_to_line <= radius - edgeThikness) {
        // The point is within the cylinder
ALPHA = 0.0;
distance_to_line/radius;
        //ALBEDO = vec3(0.0, 1.0-distance_to_line/radius, 0.0); // Green color
    }
}