To activate this function, select Select by Numbers... from Edit menu.

Select by Numbers Dialogue.

Select by Numbers

Here you may select a rectangle region of mosaic by entering selection dimensions. This function is handy when you want to render some special region with particular library. Also you may want to export specific part of mosaic into image file.

Indeed, you may select a region with dragging mouse, but it may be difficult, especial when you work with large mosaic and/or need to keep precise selection sizes.

Enter Left / Bottom coordinates of selection origin and Width / Height of selection. To active selection press Select Rectangle button.