CompPhoto: Chapter 7 Blending:ChatGPT

Point Process (Pixel Arithmetic)

An image operation applied independently to each pixel, such as addition, subtraction, or scaling.

Pixel Arithmetic

Adds, subtracts, or averages pixel values between images to create new effects.

Addition (Blend Mode)

Adds pixel values from two images; can brighten the result but often causes overexposure (too white).

Subtraction (Blend Mode)

Subtracts pixel values from one image from another; darkens the image and may cause clipping (too dark).

Difference (Blend Mode)

Calculates the absolute difference between two images, emphasizing areas of change.

Alpha (α)

Represents opacity or transparency; ranges from 0 (fully transparent) to 1 (fully opaque).

Alpha Blending (α-Blending)

A weighted combination of two images defined as f_blend(a,b) = αa + (1−α)b.

Opacity vs. Transparency

Opacity describes how visible a layer is; transparency describes how much of the lower layer shows through.

Simple Average Blending

Averages corresponding pixels from two images: f_blend(a,b) = (a + b)/2.

Normal Blend Mode

Simply replaces the pixel from one image with the pixel from another; equivalent to f_blend(a,b) = b.

Multiply Blend Mode

Darkens an image by multiplying pixel values: f_blend(a,b) = a × b.

Screen Blend Mode

Brightens an image by inverting, multiplying, and inverting again: f_blend(a,b) = 1 − (1 − a)(1 − b).

Overlay Blend Mode

Combines Multiply and Screen; bright areas become brighter, dark areas become darker.

Overlay Formula

f_blend(a,b) = 2ab if a < 0.5, otherwise 1 − 2(1 − a)(1 − b).

Dodge and Burn

Techniques used to selectively lighten (dodge) or darken (burn) areas of an image.

Dodge Mode

Lightens an image by increasing exposure in bright regions, often based on Screen mode.

Burn Mode

Darkens an image by reducing exposure in specific areas, based on Multiply mode.

Arithmetic Blend Modes

Include add, subtract, multiply, divide, difference, lighten, and darken; each affects pixel intensity differently.

Lighten Blend Mode

Chooses the maximum pixel value between two images for each channel: f_blend(a,b) = max(a,b).

Darken Blend Mode

Chooses the minimum pixel value between two images for each channel: f_blend(a,b) = min(a,b).

Divide Blend Mode

Brightens an image by dividing pixel values of one image by another; can wash out details.

Practical Application of Blending

Used for image compositing, HDR combination, cross-fading transitions, and artistic effects.

Transparency in Digital Images

Often implemented using an alpha channel in RGBA images to control layer visibility.

Importance of Blending in Computational Photography

Blending allows seamless integration of multiple images, crucial for panoramas, HDR, and exposure fusion.