Transcript Painting - Octagon Software

Painting
(Chapter 12)
Java Certification Study Group
January 25, 1999
Mark Roth
Preview
• The paint() method and the graphics
context.
• the GUI thread and the repaint() method.
• Spontaneous painting.
• Painting to images.
The paint() method and the
Graphics Context
• public void paint( Graphics g
)
• Defined in java.awt.Component
• You provide the method, AWT “knows” when
to call it:
“This method is called when the contents of the component
should be painted in response to the component first being
shown or damage needing repair. The clip rectangle in the
Graphics parameter will be set to the area which needs to
be painted.” - JDK 1.1 Javadoc
Graphics Context
(java.awt.Graphics)
• Represents the “thing” you are painting to:
– Component
– Image
– Printer
• Most Useful For:
–
–
–
–
Applet
Canvas
Frame
Panel
Selecting a Color
• Call g.setColor( color )
• Color can be one of the constants such as
Color.blue, Color.magenta, … (These are
final static Color objects)
• You can also create your own color using
new Color( r, g, b )
• Color change will be applied to all
subsequent painting operations.
Selecting a Font
• First, create the Font object:
new Font( String fontname, int style, int size )
• Font Name:
– Standard fonts:
• “Serif”
• “SansSerif”
• “Monospaced”
– Can be retrieved:
• String fontnames[] =
Toolkit.getDefaultToolkit().getFontList()
Selecting a Font, Continued
• Style:
–
–
–
–
Font.PLAIN
Font.BOLD
Font.ITALIC
Font.BOLD + Font.ITALIC
• Size: In points (e.g. 24 point)
Drawing and Filling
• Coordinate system
– X increases as you go right
– Y increases as you go down
– Upper-left Corner is (0,0)
• Defines primitive painting methods
Primitive Painting Methods
(pp. 331-338)
•
•
•
•
•
•
•
•
drawLine()
drawRect() and fillRect()
drawOval() and fillOval()
drawArc() and fillArc()
drawPolygon() and fillPolygon()
drawPolyline()
drawString()
drawImage()
Clipping
• Imposes a restriction on the region that a
graphics context can modify.
• Call g.setClip( x, y, width, height )
• See Figure 12.15 and 12.16
Painting a Contained Component
• You can subclass Canvas and provide your
own implementation of Paint to create your
own components. (See pp. 342-343)
• paint( Graphics g ) will be called at
appropriate times.
• Java will automatically paint background of
component with backgroundColor and call
setColor( foregroundColor ) before calling
paint.
Spontaneous Painting
• GUI Thread Calls Paint Under 4
Circumstances:
–
–
–
–
“After exposure
After de-iconification
Shortly after init() returns (applets only)
When a browser returns to a previously
displayed page containing an applet, provided
the applet is at least partially visible.”
Spontaneous Painting
• Rule of Thumb: Do all drawing operations
in paint()
• Call paint() to paint the contents of the
window.
• Call update() to erase the background and
then paint the contents of the window.
• Call repaint() to schedule a call to update
(avoids duplicate calls to update).
Images
• “Images are off-screen representations of
rectangular pixel patterns.”
• Operations:
– Create
– Modify
– Draw to screen or other images
Creating Images
• Create an Empty Image:
– Image im1 = component.createImage( 400, 250 )
• Create from .gif or .jpeg (in Applet or from
Toolkit):
– getImage( URL fileURL )
– getImage( URL dirURL, String path )
Drawing to Images
• Call im.getGraphics() and paint to that
context.
• Call g.drawImage( im, x, y, this ) from the
paint() method of a Component to draw the
image to the screen.
• Very useful for flicker-free doublebuffering.
References
• All Material in this Presentation is heavily
based on:
Roberts, Simon and Heller, Philip, Java 1.1
Certification Study Guide, 1997:
SYBEX™. ISBN: 0-7821-2069-5
• Selected portions from JDK 1.1.6 JavaDoc
HTML pages.