THotPDF.RegisterCoonsPatchMesh Method

Declares a PDF 1.3+ Coons patch mesh shading (ISO 32000-1 8.7.4.5.6, /ShadingType 6) and wraps it in an inline Pattern Type 2 entry on the page Resources/Pattern dictionary. Each patch is bounded by four cubic Bezier curves and carries one colour per corner; the renderer fits the Coons surface between the four edges, giving an arbitrarily curved colour-bearing quad. Use this when straight-edge triangle meshes (RegisterFreeFormGouraudShading, RegisterLatticeFormGouraudShading) would need many triangles to approximate curved boundaries - foil and metallic gradients on bent paths, SVG-derived gradient meshes, and any quad with curved edges.

function RegisterCoonsPatchMesh(XMin, YMin, XMax, YMax: Single; NumComponents: Integer; const Patches: array of Extended): AnsiString;

AnsiString RegisterCoonsPatchMesh(float XMin, float YMin, float XMax, float YMax, int NumComponents, const Extended* Patches, int PatchesElementCount);

The shading is emitted as an indirect stream with /ShadingType 6, /BitsPerCoordinate 16, /BitsPerComponent 8 and /BitsPerFlag 8. Each patch is packed as 1 flag byte (always 0 for independent patches) followed by 12 control points (each 4 bytes: 2 big-endian X + 2 big-endian Y) and 4 corner colours (each NumComponents bytes), totalling 1 + 48 + 4 * NumComponents bytes per patch. The /Decode array maps the 16-bit coordinates back to the supplied [XMin, XMax] x [YMin, YMax] user-space rectangle and each colour component back to [0, 1].

The 12 control points are numbered c1..c12 in clockwise order around the patch boundary starting from corner 1. The four corners are c1 / c4 / c7 / c10, and the remaining eight (c2, c3 between c1 and c4; c5, c6 between c4 and c7; c8, c9 between c7 and c10; c11, c12 between c10 and c1) are the inner Bezier handles of each cubic edge. When the inner handles sit at 1/3 and 2/3 along the straight line between adjacent corners, the Coons surface degenerates to bilinear interpolation - visually equivalent to a 4-vertex Type 4 mesh.

XMin, YMin, XMax, YMax - the user-space rectangle that bounds the encoded coordinates. These four values become the first four entries of /Decode. Control points outside this rectangle are clamped at encoding time. Place them in the same user-space coordinates you will paint into - for HotPDF's screen-coordinate Rectangle at (X, Y, W, H) the matching PDF user-space rectangle is (X, PageHeight - Y - H) to (X + W, PageHeight - Y).

NumComponents - number of colour components per corner. Pass 1 for DeviceGray, 3 for DeviceRGB or 4 for DeviceCMYK. Any other value raises an exception.

Patches - flat array of Extended values. Each patch contributes 24 + 4 * NumComponents values: 24 floats for the 12 control points (X, Y, X, Y, ..., 12 pairs in c1..c12 clockwise order) followed by 4 * NumComponents floats for the corner colours (corner 1 / 2 / 3 / 4 = c1 / c4 / c7 / c10 in cyclic order). Array length must be a positive multiple of this stride. All emitted patches carry flag = 0 (independent patch); continuation flags 1 / 2 / 3 (edge sharing with the previous patch) are not exposed in this convenience overload.

Return value: an auto-generated pattern name (Sh1, Sh2, ...) registered on the current page's Resources/Pattern dictionary. Returns an empty string when StrictVersionLock is on and the active Version is below PDF 1.3 (otherwise the document version auto-bumps to 1.3).

Code Example

// Single straight-edge Coons patch covering a 200x200 quad. Inner
// control points placed at 1/3 / 2/3 along each edge => bilinear
// surface. Four corner colours = red / green / yellow / blue.
var
  Doc: THotPDF;
  PatName: AnsiString;
  Patches: array[0..35] of Extended;   // 12 ctrl pts * 2 + 4 corners * 3 = 36
  H, XMin, YMin, XMax, YMax, W: Single;
begin
  Doc := THotPDF.Create(nil);
  try
    Doc.Version := pdf14;
    Doc.BeginDoc;
    H := Doc.CurrentPage.Height;
    XMin := 0; YMin := H - 200; XMax := 200; YMax := H;
    W := 200;
    // c1..c12 clockwise. Inner handles at 1/3 / 2/3 => straight edges.
    Patches[ 0] := XMin;            Patches[ 1] := YMax;             // c1  TL
    Patches[ 2] := XMin + W / 3;    Patches[ 3] := YMax;             // c2
    Patches[ 4] := XMin + 2*W / 3;  Patches[ 5] := YMax;             // c3
    Patches[ 6] := XMax;            Patches[ 7] := YMax;             // c4  TR
    Patches[ 8] := XMax;            Patches[ 9] := YMax - W / 3;     // c5
    Patches[10] := XMax;            Patches[11] := YMax - 2*W / 3;   // c6
    Patches[12] := XMax;            Patches[13] := YMin;             // c7  BR
    Patches[14] := XMax - W / 3;    Patches[15] := YMin;             // c8
    Patches[16] := XMax - 2*W / 3;  Patches[17] := YMin;             // c9
    Patches[18] := XMin;            Patches[19] := YMin;             // c10 BL
    Patches[20] := XMin;            Patches[21] := YMin + W / 3;     // c11
    Patches[22] := XMin;            Patches[23] := YMin + 2*W / 3;   // c12
    // 4 corner colours (c1 / c4 / c7 / c10).
    Patches[24] := 1.0; Patches[25] := 0.0; Patches[26] := 0.0;  // red
    Patches[27] := 0.0; Patches[28] := 1.0; Patches[29] := 0.0;  // green
    Patches[30] := 1.0; Patches[31] := 1.0; Patches[32] := 0.0;  // yellow
    Patches[33] := 0.0; Patches[34] := 0.0; Patches[35] := 1.0;  // blue
    PatName := Doc.RegisterCoonsPatchMesh(XMin, YMin, XMax, YMax, 3, Patches);
    Doc.CurrentPage.SetFillPattern(PatName);
    Doc.CurrentPage.Rectangle(0, 0, 200, 200);
    Doc.CurrentPage.Fill;
    Doc.EndDoc;
  finally
    Doc.Free;
  end;
end;