Meziantou.FrameworkFilter Class
An effect applied to a drawing as a whole, or to what is behind it: a blur, a color matrix, a drop shadow, or a chain of them, as the filter and backdrop-filter properties of CSS.
public abstract class FilterRemarks
A filter is applied by DrawingCanvas.Filter, to what a callback draws, and by DrawingCanvas.BackdropLayer, to what is already drawn behind a rectangle. Its lengths (the deviation of a blur, the offset of a shadow) are lengths of the drawing: a transformation of the canvas that enlarges the drawing enlarges them.
Arithmetic. The pixels are read as 16-bit colors and filtered as colors multiplied by their alpha, as fractions of one, kept in single precision between two filters of a chain and summed in double precision. The result is rounded to the samples of the layer it is written to: the alpha as floor(M a + 1/2 + 2^-20) and each color as floor(M p / a + 1/2 + 2^-20), with M the largest sample value. Filters are immutable and thread-safe.
Methods
| Name | Description |
|---|---|
| TryGetBounds(RectangleD, RectangleD) | Gets a rectangle that contains everything the filter paints when what it filters stays inside a rectangle: how far a blur or a shadow spreads, to size an image or a layer that keeps all of it. |
TryGetBounds(RectangleD, RectangleD)
Filter.csGets a rectangle that contains everything the filter paints when what it filters stays inside a rectangle: how far a blur or a shadow spreads, to size an image or a layer that keeps all of it.
public bool TryGetBounds(RectangleD bounds, out RectangleD result)Parameters
boundsRectangleD- A rectangle that contains what is filtered.
resultRectangleD- When the method returns
true, a rectangle that contains what the filter paints; otherwise, an empty rectangle.
Returns
bool true when the filter paints within a rectangle; false when it can paint anywhere on the canvas.
Exceptions
ArgumentException- An edge of
boundsis not a finite number. ArgumentOutOfRangeException- The magnitude of an edge of
boundsexceeds 16777216.
Remarks
The rectangles are in the coordinates of the drawing the filter is applied to, as its lengths are: before the transformation of the canvas. Filtered pixels are whole pixels: the pixels the result changes are within the rectangle rounded out to whole pixels.
A blur spreads by three times its standard deviation, rounded up; a drop shadow covers what it filters (unless DropShadowFilter.ShadowOnly) and the same rectangle moved by its offset and spread by its blur; a color matrix stays in the rectangle; a chain combines the rectangles of its filters; a filter graph stays in its FilterGraph.Region.
Some filters paint where nothing is drawn, so nothing bounds them but the canvas: a color matrix that adds to the alpha, a blur whose BlurFilter.EdgeMode reads the edges of the canvas, and a filter graph without a region (a flood or a turbulence fills it). The method returns false for those.