interactive picture construction techniques in computer graphics

interactive picture construction techniques in computer graphics
Basic Concept:
There are many techniques that are added into graphics packages to help the interactive construction of pictures.
Various input options can be provided, so that coordinate information entered with locator and stroke devices can be adjusted or interpreted according to a selected option.
For example, we can restrict all lines to be either horizontal or vertical.
1. Constraints
A constraint is a rule for altering input-coordinate values to produce a specified orientation or alignment of the displayed coordinates.
There are many kinds of constraint functions, but the most common constraint are horizontal alignment or vertical alignment of straight lines.
2. Basic Positioning Methods
coordinate values supplied by locator input are used with positioning methods to specify a location for displaying an object or a character string.
Coordinate positions are selected interactively with a pointing device, usually by positioning the screen cursor. 
3. GRAVITY FIELD
In gravity field, when we need to place one end point of a line on other line, we don’t need to be precise in our cursor placement as it would be really difficult to be accurate.
Hence the software package adds a function called “Gravity field” near the line so that whenever any end point is brought near the area of our line, its placed ON the line.
The “Gravity field” is covers enough area to allow accurate placement on line but, not so much such that it affects other lines gravity field.
4. Grids
A grid is another constraint which is really useful for accurate placement of objects on the screen.
Basically it places the endpoints near the closest interaction on the grid.
You will understand better with an example:
5. Rubber Band Method
In Rubber Band Method Straight line segment can be drawn and positioned using rubber band method where we anchor one end point and try different position for second end point.
It helps us get an Idea how the line segment will look like before actually placing it at the end point positions.
6. Painting and Drawing
Painting and drawing is exactly what it sounds like.
We all must have used Microsoft paint program.
We can draw a circle, rectangle, different shapes, curved lines, straight lines, in Paint.
We can fill color, change the width and other attributes.
7. Dragging
Dragging is the method or technique which is used to position objects or change the size of the objects by “clicking and holding and Dragging” either the object or one of the endpoint of the object.
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