Function ncplane_gradient 

pub fn ncplane_gradient(
    plane: &mut NcPlane,
    y: Option<u32>,
    x: Option<u32>,
    len_y: Option<u32>,
    len_x: Option<u32>,
    egc: &str,
    stylemask: impl Into<NcStyle_u16>,
    ul: impl Into<NcChannels_u64>,
    ur: impl Into<NcChannels_u64>,
    ll: impl Into<NcChannels_u64>,
    lr: impl Into<NcChannels_u64>,
) -> NcResult_i32

Draws a gradient with its upper-left corner at the current cursor position, stopping at end_y×end_x.

Use None for either or all of beg_y and beg_x in order to use the current cursor position along that axis.

Use None for either or both of len_y and len_x in order to go through the boundary of the plane in that axis (same as 0).

The glyph composed of egc and stylemask is used for all cells. The NcChannels_u64 specified by ul, ur, ll, and lr are composed into foreground and background gradients.

• To do a vertical gradient, ul ought equal ur and ll ought equal lr.
• To do a horizontal gradient, ul ought equal ll and ur ought equal ul.
• To color everything the same, all four channels should be equivalent. The resulting alpha values are equal to incoming alpha values. Returns the number of cells filled on success, or -1 on failure.

Palette-indexed color is not supported.

Preconditions for gradient operations (error otherwise):

• all: only RGB colors, unless all four channels match as default
• all: all alpha values must be the same
• 1x1: all four colors must be the same
• 1xN: both top and both bottom colors must be the same (vertical gradient)
• Nx1: both left and both right colors must be the same (horizontal gradient)

Method: NcPlane.gradient().