Filters that cannot be seamlessly tiled within an image with a non-integer aspect ratio are called discrete. The most obvious example of a discrete filter is a 2x2 checkerboard pattern based on the Checker component.
In order for a discrete filter to be seamlessly tiled, the aspect ratio of the source image must be integer. The aspect ratio is the ratio of the image width to the image height, for example an image with the dimensions of 400x200 pixels has an integer aspect ratio of 2, and an image of 300x200 pixels has a non-integer aspect ratio of 1.5. For vertically oriented images with the height greater than the width (for example 200x400 pixels), the width and height get reversed so that the ratio is always greater than 1.
The difference between a discrete and a non-discrete filter is only manifested when Seamless Tiling is activated on the Settings tab in Filter Controls. For a discrete filter, the Size slider in Filter Controls moves in discrete increments and can't be set to an arbitrary value. Also, when Show Border is turned on in the filter preview and the source image has a non-integer aspect ratio, the border shows the tiling area in addition to the image boundaries. When Seamless Tiling is turned off, discrete filters behave just like other filters.
When a discrete filter with Seamless Tiling turned on is applied to an image with a non-integer aspect ratio, Filter Forge shows a notice that the output image might be non-seamless. This notification can be turned off in Interface Options.
A filter becomes discrete when it uses at least one discrete component, such as Bricks, Pavements, Tiles, 3-Color Gradient, 5-Color Gradient, Profile Gradient, Checker or Kaleidoscope.
