Using Color in Direct2D
Direct2D uses the RGB color model, in which colors are formed by combining different values of red, green, and blue. A fourth component, alpha, measures the transparency of a pixel. In Direct2D, each of these components is a floating-point value with a range of [0.0 1.0]. For the three color components, the value measures the intensity of the color. For the alpha component, 0.0 means completely transparent, and 1.0 means completely opaque. The following table shows the colors that result from various combinations of 100% intensity.
Color values between 0 and 1 result in different shades of these pure colors. Direct2D uses the D2D1_COLOR_F structure to represent colors. For example, the following code specifies magenta.
// Initialize a magenta color. D2D1_COLOR_F clr; clr.r = 1; clr.g = 0; clr.b = 1; clr.a = 1; // Opaque.
// Equivalent to the previous example. D2D1::ColorF clr(1, 0, 1, 1);
Alpha blending creates translucent areas by blending the foreground color with the background color, using the following formula.
- color = af Cf + (1 af) Cb
where Cb is the background color, Cf is the foreground color, and af is the alpha value of the foreground color. This formula is applied pairwise to each color component. For example, suppose the foreground color is (R = 1.0, G= 0.4, B = 0.0), with alpha = 0.6, and the background color is (R = 0.0, G = 0.5, B = 1.0). The resulting alpha-blended color is:
- R = (1.0 0.6 + 0 0.4) = .6
G = (0.4 0.6 + 0.5 0.4) = .44
B = (0 0.6 + 1.0 0.4) = .40
The following image shows the result of this blending operation.
The D2D1_COLOR_F structure does not describe how a pixel is represented in memory. In most cases, that doesn't matter. Direct2D handles all of the internal details of translating color information into pixels. But you might need to know the pixel format if you are working directly with a bitmap in memory, or if you combine Direct2D with Direct3D or GDI.
The DXGI_FORMAT enumeration defines a list of pixel formats. The list is fairly long, but only a few of them are relevant to Direct2D. (The others are used by Direct3D).
||This is the most common pixel format. All pixel components (red, green, blue, and alpha) are 8-bit unsigned integers. The components are arranged in BGRA order in memory. (See illustration that follows.)
||Pixel components are 8-bit unsigned integers, in RGBA order. In other words, the red and blue components are swapped, relative to DXGI_FORMAT_B8G8R8A8_UNORM. This format is supported only for hardware devices.
||This format contains an 8-bit alpha component, with no RGB components. It is useful for creating opacity masks. To read more about using opacity masks in Direct2D, see Compatible A8 Render Targets Overview.
The following illustration shows BGRA pixel layout.
To get the pixel format of a render target, call ID2D1RenderTarget::GetPixelFormat. The pixel format might not match the display resolution. For example, the display might be set to 16-bit color, even though the render target uses 32-bit color.
A render target also has an alpha mode, which defines how the alpha values are treated.
|D2D1_ALPHA_MODE_IGNORE||No alpha blending is performed. Alpha values are ignored.|
|D2D1_ALPHA_MODE_STRAIGHT||Straight alpha. The color components of the pixel represent the color intensity prior to alpha blending.|
|D2D1_ALPHA_MODE_PREMULTIPLIED||Premultiplied alpha. The color components of the pixel represent the color intensity multiplied by the alpha value. This format is more efficient to render than straight alpha, because the term (af Cf) from the alpha-blending formula is pre-computed. However, this format is not appropriate for storing in an image file.|
Here is an example of the difference between straight alpha and premultiplied alpha. Suppose the desired color is pure red (100% intensity) with 50% alpha. As a Direct2D type, this color would be represented as (1, 0, 0, 0.5). Using straight alpha, and assuming 8-bit color components, the red component of the pixel is 0xFF. Using premultiplied alpha, the red component is scaled by 50% to equal 0x80.
The D2D1_COLOR_F data type always represents colors using straight alpha. Direct2D converts pixels to premultiplied alpha format if needed.
If you know that your program will not perform any alpha blending, create the render target with the D2D1_ALPHA_MODE_IGNORE alpha mode. This mode can improve performance, because Direct2D can skip the alpha calculations. For more information, see Improving the Performance of Direct2D Applications.