Stage doesn't show obstacles

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Hi everyone, I am using stage to simulate robot exploration and mapping. I have some maps that are both(same map) with or without obstacles(furnitures). But when I put the png in stage it doesn't see the obstacles because they are "less black" than the walls. I was wondering if there is a parameter in stage that tells him if a point is a obstacle or it is nothing. I didn't find anything in the documentation. Otherwise I have to darken all the maps I think.