IT 9626 - Graphics Creation

19. Edit Bitmap Images (Selection, Correction, Resizing)

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

1. Selection Techniques

Selection allows you to isolate specific areas of an image for targeted editing. Various selection tools are available in image editing software.

• Rectangular Marquee Tool: Selects rectangular areas.
• Elliptical Marquee Tool: Selects elliptical or circular areas.
• Lasso Tool: Allows freehand selection by drawing a path.
• Polygonal Lasso Tool: Creates selections based on straight lines.
• Magnetic Lasso Tool: Attempts to automatically snap to edges based on color contrast.
• Magic Wand Tool: Selects pixels with similar color values.

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

2. Image Correction

Image correction involves adjusting the visual properties of an image to improve its quality.

2.1 Brightness and Contrast

Adjusting brightness increases or decreases the overall lightness of the image. Adjusting contrast enhances the difference between light and dark areas.

2.2 Colour Correction

Colour correction allows for adjustments to the colour balance of an image.

• Hue: Changes the dominant colour.
• Saturation: Controls the intensity of the colours.
• Lightness: Adjusts the lightness or darkness of the colours.

2.3 Other Corrections

Other common corrections include:

• Levels: Adjusts the tonal range of the image.
• Curves: Provides more precise control over tonal adjustments.
• Sharpening: Enhances the edges and details in the image.
• Blurring: Reduces sharpness and detail.

3. Image Resizing

Resizing changes the dimensions of an image. This can affect the image quality.

3.1 Resizing Methods

Image editing software offers various methods for resizing:

• Nearest Neighbour: Simplest method, can result in pixelated images.
• Bilinear: Produces smoother results than nearest neighbour.
• Bicubic: Generally provides the best quality for resizing.

3.2 Aspect Ratio

Maintaining the aspect ratio ensures the image is not distorted during resizing. Aspect ratio is the ratio of width to height.

3.3 Resizing Considerations

When resizing, consider the intended use of the image. Increasing the size of an image can lead to pixelation. Reducing the size might lose detail.