It sounds like your layer mask is all black (opaque). So when the invert box is off – everything is masked. When the invert box is on, the mask will be all white (transparent) and the grid will appear everywhere. You need to make sure that the mask traces the walls. So that it is black where the walls are, and white where the floors are. I’m happy to take a look – send the gimp file to me at jonathan@fantasticmaps.com and I’ll take a look at it.

Jon

The first line is width – this sets the line width of the grid lines. You’ve currently set them to be 50px wide. I usually stick with 1px lines.

The second line is spacing – this is the distance between the grid lines. I use a 50px grid in this tutorial – so I set the spacing between the lines to be 50px.

The last line is the offset – this is the distance that the grid is offset in from the top left corner of the picture. As I’m starting from scratch and don’t want half squares at the side of the map I set the offset to be zero. You can play with these and you’ll see the grid move around. This is where you’d change the numbers to get a grid to line up with an existing map.

Everything in the third column can safely be set to zero.

So my settings would be:
W 1 1 0
S 50 50 0
O 0 0 0

with all three boxes at the bottom set to zero.

The reason that you’re example gives you all black is that you have 50px wide grid lines spaced 50px apart. That’s one black line right beside another black line beside another…. so you’ll get nothing but overlapping black lines on the screen. Everything will be black. Bring down the Width value to 1 and you’ll be fine.

H V I
W50 50 0

S50 50 0

O0 0 1