HOW TO：使用複合模式控制 Alpha 混色
有時候，您可能會想要建立具有下列特性的螢幕外點陣圖：
色彩中的 Alpha 值小於 255。
建立點陣圖時不要使色彩彼此產生 Alpha 混色。
顯示完成的點陣圖時，點陣圖中的色彩已與顯示裝置上的背景色彩產生 Alpha 混色。
若要建立此類點陣圖，請先建構空白的 Bitmap 物件，然後再根據該點陣圖建構 Graphics 物件。 請將 Graphics 物件的複合模式設定為 CompositingMode.SourceCopy。
範例
下列範例根據 Bitmap 物件來建立 Graphics 物件。 程式碼使用 Graphics 物件和兩個半透明筆刷 (Alpha = 160) 在點陣圖上進行塗繪。 程式碼會使用半透明的筆刷來填滿紅色橢圓形和綠色橢圓形。 綠色橢圓形與紅色橢圓形重疊，但是因為 Graphics 物件的複合模式已設定為 SourceCopy，因此綠色和紅色不會混合。
程式碼會在螢幕上繪製兩次點陣圖：一次是在白色背景上，一次是在多種色彩的背景上。 屬於這兩個橢圓形一部分的點陣圖中的像素具有 Alpha 元素值 160，因此橢圓形會與螢幕上的背景色彩混合。
下圖顯示的是程式碼範例的輸出。 請注意，橢圓形會與背景混合，但是不會彼此混合。
.png "原始檔複製")
程式碼範例中包含此陳述式：
bitmapGraphics.CompositingMode = CompositingMode.SourceCopy
bitmapGraphics.CompositingMode = CompositingMode.SourceCopy;
如果想要將橢圓形彼此混合，同時也要與背景混合，請將上述陳述式變更如下：
bitmapGraphics.CompositingMode = CompositingMode.SourceOver
bitmapGraphics.CompositingMode = CompositingMode.SourceOver;
下圖顯示的是修改程式碼後的輸出。
.png "原始檔停留")
' Create a blank bitmap.
Dim myBitmap As New Bitmap(180, 100)
' Create a Graphics object that we can use to draw on the bitmap.
Dim bitmapGraphics As Graphics = Graphics.FromImage(myBitmap)
' Create a red brush and a green brush, each with an alpha value of 160.
Dim redBrush As New SolidBrush(Color.FromArgb(160, 255, 0, 0))
Dim greenBrush As New SolidBrush(Color.FromArgb(160, 0, 255, 0))
' Set the compositing mode so that when we draw overlapping ellipses,
' the colors of the ellipses are not blended.
bitmapGraphics.CompositingMode = CompositingMode.SourceCopy
' Fill an ellipse using a red brush that has an alpha value of 160.
bitmapGraphics.FillEllipse(redBrush, 0, 0, 150, 70)
' Fill a second ellipse using a green brush that has an alpha value of
' 160. The green ellipse overlaps the red ellipse, but the green is not
' blended with the red.
bitmapGraphics.FillEllipse(greenBrush, 30, 30, 150, 70)
'Set the compositing quality of the form's Graphics object.
e.Graphics.CompositingQuality = CompositingQuality.GammaCorrected
' Draw a multicolored background.
Dim colorBrush As New SolidBrush(Color.Aqua)
e.Graphics.FillRectangle(colorBrush, 200, 0, 60, 100)
colorBrush.Color = Color.Yellow
e.Graphics.FillRectangle(colorBrush, 260, 0, 60, 100)
colorBrush.Color = Color.Fuchsia
e.Graphics.FillRectangle(colorBrush, 320, 0, 60, 100)
'Display the bitmap on a white background.
e.Graphics.DrawImage(myBitmap, 0, 0)
' Display the bitmap on a multicolored background.
e.Graphics.DrawImage(myBitmap, 200, 0)
// Create a blank bitmap.
Bitmap myBitmap = new Bitmap(180, 100);
// Create a Graphics object that we can use to draw on the bitmap.
Graphics bitmapGraphics = Graphics.FromImage(myBitmap);
// Create a red brush and a green brush, each with an alpha value of 160.
SolidBrush redBrush = new SolidBrush(Color.FromArgb(160, 255, 0, 0));
SolidBrush greenBrush = new SolidBrush(Color.FromArgb(160, 0, 255, 0));
// Set the compositing mode so that when we draw overlapping ellipses,
// the colors of the ellipses are not blended.
bitmapGraphics.CompositingMode = CompositingMode.SourceCopy;
// Fill an ellipse using a red brush that has an alpha value of 160.
bitmapGraphics.FillEllipse(redBrush, 0, 0, 150, 70);
// Fill a second ellipse using a green brush that has an alpha value of 160.
// The green ellipse overlaps the red ellipse, but the green is not
// blended with the red.
bitmapGraphics.FillEllipse(greenBrush, 30, 30, 150, 70);
// Set the compositing quality of the form's Graphics object.
e.Graphics.CompositingQuality = CompositingQuality.GammaCorrected;
// Draw a multicolored background.
SolidBrush colorBrush = new SolidBrush(Color.Aqua);
e.Graphics.FillRectangle(colorBrush, 200, 0, 60, 100);
colorBrush.Color = Color.Yellow;
e.Graphics.FillRectangle(colorBrush, 260, 0, 60, 100);
colorBrush.Color = Color.Fuchsia;
e.Graphics.FillRectangle(colorBrush, 320, 0, 60, 100);
// Display the bitmap on a white background.
e.Graphics.DrawImage(myBitmap, 0, 0);
// Display the bitmap on a multicolored background.
e.Graphics.DrawImage(myBitmap, 200, 0);
編譯程式碼
上述範例是專為與 Windows Form 搭配使用而設計的，而且它需要 PaintEventArgs e (即 PaintEventHandler 的參數)。