Add Shadow

DsImaging provides ApplyGaussianBlur and ToShadowBitmap methods in GrayscaleBitmap class. These methods make the process of drawing an image with a shadow easier and more straightforward. The ApplyGaussianBlur method accepts the borderColor, radius, and borderMode arguments. The ToShadowBitmap method simplifies moving a transparency mask from GrayscaleBitmap to a GcBitmap, and has the ability to pass the shadowColor and opacity factor. Also, the ToShadowBitmap always treats the GrayscaleBitmap as a transparency mask, even if it was created from any color channel (not necessarily the alpha channel).

To add a shadow to an image:

Draw an image with some shapes and text.

// Initialize GcBitmap.
using var bmp = new GcBitmap(800, 600, false);

// Draw an image.
using (var g = bmp.CreateGraphics(Color.AliceBlue))
{
    Draw(g, 0, 0);
}

// Save the image without a shadow.
bmp.SaveAsPng("WithoutSahdow.png");

static void Draw(GcGraphics g, float offsetX, float offsetY)
{
    // Define the transformation matrix.
    var baseT = Matrix3x2.CreateTranslation(offsetX, offsetY);
    g.Transform = baseT;

    // Draw an ellipse.
    g.DrawEllipse(new RectangleF(100, 100, 300, 200),
        new Pen(Color.Orange, 20f));

    // Draw a line.
    g.DrawLine(new PointF(50, 400), new PointF(500, 50),
        new Pen(Color.RoyalBlue, 20f)
        {
            LineCap = PenLineCap.Round
        });

    // Draw strings.
    g.DrawString("Shadow",
        new TextFormat
        {
            FontName = "Segoe UI",
            FontSize = 40,
            ForeColor = Color.MistyRose,
            StrokePen = new Pen(Color.DarkRed, 1f)
        },
        new PointF(200, 150));
    g.Transform = Matrix3x2.CreateRotation((float)(Math.PI / 6)) *
        (Matrix3x2.CreateTranslation(50, 250) * baseT);
    g.DrawString("The shadow is added to both text and shapes.",
        new TextFormat
        {
            FontName = "Times New Roman",
            FontSize = 18,
            ForeColor = Color.CornflowerBlue
        },
        new PointF(0, 0));

    // Draw a rectangle.
    g.DrawRectangle(new RectangleF(-15, -10, 470, 50),
        new Pen(Color.Salmon, 1f));
}

Draw the image on a transparent background with an offset for the shadow.

// Draw the image to the transparent background with an offset for shadow.
using (var g = bmp.CreateGraphics(Color.Transparent))
{
    Draw(g, 20, 50);
}

Extract the alpha channel from GcBitmap to a GrayscaleBitmap.

// Extract the alpha channel from GcBitmap to a GrayscaleBitmap.
using var gs = bmp.ToGrayscaleBitmap(ColorChannel.Alpha);

Apply some blur to the GrayscaleBitmap using the ApplyGaussianBlur method.
```
// Apply some blur to GrayscaleBitmap.
gs.ApplyGaussianBlur(9);
```
Convert the transparency mask from GrayscaleBitmap to GcBitmap, filling the opaque pixels with the shadow color. Draw the transparency mask into the same bitmap using the ToShadowBitmap; there is no need to create another GcBitmap instance.
```
// Convert the transparency mask from GrayscaleBitmap to GcBitmap. Apply an additional opacity factor.
gs.ToShadowBitmap(bmp, Color.CadetBlue, 0.4f);
```

Substitute the transparent background with an opaque background color.

// Substitute the transparent background with an opaque background color.
bmp.ConvertToOpaque(Color.AliceBlue);

Draw the main image onto the shadow image and save the image.

// Draw the main image.
using (var g = bmp.CreateGraphics())
{
    Draw(g, 0f, 0f);
}

// Save the image with a shadow.
bmp.SaveAsPng("WithSahdow.png");