This function blends two images together using their alpha channels.

Declaration:

Arguments:

Name	Type	Description
hSource	HIGEAR	First image for blending. It may or may not contain an alpha channel.
hAlpha	HIGEAR	Second image for blending. It must contain an alpha channel.
blendingArea	LPAT_RECT	Rectange area of the first image for blending. NULL means the entire image.

Return Value:

Returns 0 if successful. Otherwise, returns the number of ImageGear errors that occurred during this function call.

Supported Raster Image Formats:

hSource:

• Grayscale - 2..16 bpp;
• RGB - 6..48 bpp;
• GyA - 4..32 bpp;
• GyPA - 4..32 bpp;
• RGBA - 8..64 bpp;
• RGBPA - 8..64 bpc.

hAlpha:

• GyA - 4..32 bpp;
• GyPA - 4..32 bpp;
• RGBA - 8..64 bpp;
• RGBPA - 8..64 bpc.

Sample:

Alpha

Remarks:

hAlpha is blended over hSource, and the result is stored in hSource. hAlpha should contain an Alpha channel, otherwise, an error is returned. Color channels (all channels except Extra and Alpha/Premultiplied Alpha) of both images must have the same color space and the same bit depths.

If hSource does not contain an Alpha channel, hAlpha is composited over it using hSource as the background. The resulting image does not contain an Alpha channel. The following formulas are used:

• If Page2 is not pre-multiplied,
• I1 = I2 * A2 + I1 (1-A2)
• If Page2 is pre-multiplied,
• I1 = I2 + I1 (1-A2)

If hSource contains an Alpha channel, hSource and hAlpha are blended together. The resulting image contains an alpha channel. Blending two images, both of which contain an Alpha channel, can be interpreted as placing one semi-transparent film over another semi-transparent film. The result of applying such combined images to some background is the same as applying one image over background and then applying another image to the result. The code

• IG_image_blend_with_alpha(hImage1, hImage2, NULL);
• IG_image_blend_with_alpha(hBackground, hImage1, NULL);

will produce the same result as

• IG_image_blend_with_alpha(hBackground, hImage1, NULL);
• IG_image_blend_with_alpha(hBackground, hImage2, NULL);

The following pseudocode demonstrates the formulas used by the function:

• if (A1=A2=0)
• I1 = 0; A1 = 0;
• else
• If both Page1 and Page2 are not pre-multiplied,
• I1 = (I2*A2 + I1*A1*(1-A2)) / (A1 + A2 - A1*A2)
• A1 = A1 + A2 - A1*A2
• //Result is not pre-multiplied
• If Page1 is not pre-multiplied, and Page2 is pre-multiplied:
• I1 = (I2 + I1*A1*(1-A2)) / (A1 + A2 - A1*A2)
• A1 = A1 + A2 - A1*A2
• //Result is not pre-multiplied
• If Page1 is pre-multiplied, and Page2 is not:
• I1 = I2*A2 + I1*(1-A2)
• A1 = A1 + A2 - A1*A2
• //Result is pre-multiplied
• If both Page1 and Page2 are pre-multiplied:
• I1 = I2 + I1*(1-A2)
• A1 = A1 + A2 - A1*A2
• //Result is pre-multiplied

Alpha values are mapped to a float value between 0.0 and 1.0, where 0.0 means full transparency and 1.0 means full opaquity. Alpha value of 0 corresponds to float value of 0.0 and alpha value of 2^n-1 (where n is alpha channel's bit depth) corresponds to float value of 1.0.

This function does not process Extra Channels.