19. Graphics Creation: Editing Bitmap Images

This section details the techniques for editing bitmap images, covering selection, correction, and resizing. Bitmap images are composed of a grid of pixels, and editing involves manipulating these individual pixels.

1. Image Selection

Selection is a fundamental operation in image editing. It allows you to isolate specific areas of an image for targeted modifications. Various selection tools are available, each suited for different types of selections.

• Rectangular Marquee Tool: Selects rectangular areas.
• Elliptical Marquee Tool: Selects elliptical areas.
• Lasso Tool: Allows freehand selection of an area.
• Polygonal Lasso Tool: Creates selections based on straight lines.
• Magnetic Lasso Tool: Attempts to automatically snap to edges based on color contrast.
• Quick Selection Tool: Quickly selects areas based on color and texture.

Once a selection is made, the selected area can be manipulated independently of the rest of the image.

2. Image Correction

Image correction involves adjusting the visual properties of an image to improve its appearance. This can include adjustments to brightness, contrast, color balance, and sharpness.

Correction Type	Description	Example
Brightness/Contrast	Adjusts the overall lightness and difference between light and dark areas.	Increasing brightness to lighten a dark image; increasing contrast to make colors more vivid.
Color Balance	Adjusts the levels of red, green, and blue in an image.	Correcting a color cast (e.g., a yellowish tint) or creating a specific color mood.
Hue/Saturation	Modifies the colors in an image, including their intensity and purity.	Increasing saturation to make colors more vibrant; changing the hue of an object.
Levels	Adjusts the black point, white point, and midtones of an image.	Improving the dynamic range of an image by expanding the range of visible tones.
Curves	Provides more precise control over tonal adjustments by manipulating curves.	Creating complex tonal adjustments for specific areas of an image.

3. Image Resizing

Resizing an image involves changing its dimensions (width and height). This can be necessary for various purposes, such as preparing images for web use or printing.

When resizing, it's important to consider the aspect ratio (the ratio of width to height) to avoid distortion. Different resampling methods can be used to determine how pixels are recalculated during resizing.

Resampling Method	Description	Typical Use
Nearest Neighbor	The simplest method; uses the nearest pixel value. Can result in pixelated images.	Quick resizing where quality is not critical.
Bilinear	Calculates a weighted average of the four nearest pixels. Produces smoother results than nearest neighbor.	General-purpose resizing.
Bicubic	Uses a more complex algorithm to calculate a weighted average of 16 surrounding pixels. Generally produces the best quality results.	Resizing images for printing or high-quality display.

Resizing can be either proportional (maintaining the aspect ratio) or non-proportional (allowing changes to width and height independently). Proportional resizing is generally preferred to avoid distortion.