SkiaSharp blend modes
These articles focus on the BlendMode property of SKPaint. The BlendMode property is of type SKBlendMode, an enumeration with 29 members.
The BlendMode property determines what happens when a graphical object (often called the source) is rendered on top of existing graphical objects (called the destination). Normally, we expect the new graphical object to obscure the objects underneath it. But that happens only because the default blend mode is SKBlendMode.SrcOver, which means that the source is drawn over the destination. The other 28 members of SKBlendMode cause other effects. In graphics programming, the technique of combining graphical objects in various ways is known as compositing.
The SKBlendModes enumeration
The SkiaSharp blend modes correspond closely to those described in the W3C Compositing and Blending Level 1 specification. The Skia SkBlendMode Overview also provides helpful background information. For a general introduction to blend modes, the Blend modes article in Wikipedia is a good start. Blend modes are supported in Adobe Photoshop, so there is much additional online information about blend modes in that context.
The 29 members of the SKBlendMode enumeration can be divided into three categories:
| Porter-Duff | Separable | Non-Separable |
|---|---|---|
Clear |
Modulate |
Hue |
Src |
Screen |
Saturation |
Dst |
Overlay |
Color |
SrcOver |
Darken |
Luminosity |
DstOver |
Lighten |
|
SrcIn |
ColorDodge |
|
DstIn |
ColorBurn |
|
SrcOut |
HardLight |
|
DstOut |
SoftLight |
|
SrcATop |
Difference |
|
DstATop |
Exclusion |
|
Xor |
Multiply |
|
Plus |
The names of these three categories will take on more meaning in the discussions that follow. The order that the members are listed here is the same as in the definition of the SKBlendMode enumeration. The 13 enumeration members in the first column have the integer values 0 to 12. The second column are enumeration members that correspond to integers 13 to 24, and the members in the third column have values of 25 to 28.
These blend modes are discussed in approximately the same order in the W3C Compositing and Blending Level 1 document, but there are a few differences: The Src mode is called Copy in the W3C document, and Plus is called Lighter. The W3C document defines a Normal blend mode that isn't included in SKBlendModes because it would be the same as SrcOver. The Modulate blend mode (at the top of the second column) isn't included in the W3C document, and discussion of the Multiply mode precedes Screen.
Because the Modulate blend mode is unique to Skia, it will be discussed as an additional Porter-Duff mode and as a separable mode.
The importance of transparency
Historically, compositing was developed in conjunction with the concept of the alpha channel. In a display surface such as the SKCanvas object and a full-color bitmap, each pixel consists of 4 bytes: 1 byte each for the red, green, and blue components, and an additional byte for transparency. This alpha component is 0 for full transparency and 0xFF for full opacity, with different levels of transparency between those values.
Many of the blend modes rely on transparency. Usually, when an SKCanvas is first obtained in a PaintSurface handler, or when an SKCanvas is created to draw on a bitmap, the first step is this call:
canvas.Clear();
This method replaces all the pixels of the canvas with transparent black pixels, equivalent to new SKColor(0, 0, 0, 0) or the integer 0x00000000. All the bytes of all the pixels are initialized to zero.
The drawing surface of an SKCanvas that is obtained in a PaintSurface handler might appear to have a white background, but that's only because the SKCanvasView itself has a transparent background, and the page has a white background. You can demonstrate this fact to yourself by setting the Xamarin.Forms BackgroundColor property of SKCanvasView to a Xamarin.Forms color:
canvasView.BackgroundColor = Color.Red;
Or, in a class that derives from ContentPage, you can set the page background color:
BackgroundColor = Color.Red;
You'll see this red background behind your SkiaSharp graphics because the SkiaSharp canvas itself is transparent.
The article SkiaSharp Transparency showed some basic techniques in using transparency to arrange multiple graphics in a composite image. The blend modes go beyond that, but transparency remains crucial to the blend modes.
SkiaSharp Porter-Duff blend modes
Use the Porter-Duff blend modes to compose scenes based on source and destination images.
SkiaSharp separable blend modes
Use the separable blend modes to alter red, green, and blue colors.
SkiaSharp non-separable blend modes
Use the non-separable blend modes to alter hue, saturation, or luminosity.