Delphi 5 - Mastering Delphi 5 - Chapter 22 - Graphics.pdf - Delphi 5 - musli_com

SYBEX Supplement

Mastering™ Delphi™ 5

by Marco Cantù

Chapter 22: Graphics in Delphi

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CHAPTER

TWENTY-TWO

Graphics in Delphi

Painting on a form

Animated buttons

An image viewer

Drawing over a bitmap

Graphical grids and games

Using TeeChart

Windows metafiles

Chapter 22 • Graphics in Delphi

I n Chapter 6 of Mastering Delphi 5, I introduced the Canvas object, Windows

painting process, and the OnPaint event. In this bonus chapter, I’m going to start

from this point and continue covering graphics, following a number of different

directions. (For all the code discussed here and in Mastering Delphi 5, check the

Sybex Web site.)

I’ll start with the development of a complex program to demonstrate how the

Windows painting model works. Then I’ll focus on some graphical components,

such as graphical buttons and grids. During this part of the chapter we’ll also add

some animation to the controls.

Finally, this chapter will discuss the use of bitmaps, covering some advanced

features for fast graphics rendering, metafiles, the TeeChart component (including

its use on the Web), and few more topics related to the overall issue of graphics.

Drawing on a Form

In Chapter 6, we saw that it is possible to paint directly on the surface of a form in

response to a mouse event. To see this behavior, simply create a new form with

the following OnMouseDown event handler:

procedure TForm1.FormMouseDown(Sender: TObject;

Button: TMouseButton; Shift: TShiftState; X, Y: Integer);

begin

Canvas.Ellipse (X-10, Y-10, X+10, Y+10);

end ;

The program seems to work fairly well, but it doesn’t. Every click produces a new

circle, but if you minimize the form, they’ll all go away. Even if you cover a por-

tion of your form with another window, the shapes behind that other form will

disappear, and you might end up with partially painted circles.

As I detailed in Chapter 6, this direct drawing is not automatically supported by

Windows. The standard approach is to store the painting request in the OnMouse-

Down event and then reproduce the output in the OnPaint event. This event, in fact,

is called by the system every time the form requires repainting . However, you’ll

need to force its activation by calling the Invalidate or Repaint methods in the

mouse-event handler. In other words, Windows knows when the form has to be

repainted because of a system operation (such as placing another window in front

of your form), but your program must notify the system when painting is required

because of user input or other program operations.

Drawing on a Form

The Drawing Tools

All the output operations in Windows take place using objects of the TCanvas

class. The output operations usually don’t specify colors and similar elements but

use the current drawing tools of the canvas. Here is a list of these drawing tools

(or GDI objects, from the Graphics Device Interface, which is one of the Windows

system libraries):

•

The Brush property determines the color of the enclosed surfaces. The

brush is used to fill closed shapes, such as circles or rectangles. The proper-

ties of a brush are Color, Style, and optionally, Bitmap.

•

The Pen property determines the color and size of the lines and of the borders

of the shapes. The properties of a pen are Color, Width, and Style, which

includes several dotted and dashed lines (available only if the Width is 1

pixel). Another relevant subproperty of the Pen is the Mode property, which

indicates how the color of the pen modifies the color of the drawing surface.

The default is simply to use the pen color (with the pmCopy style), but it is also

possible to merge the two colors in many different ways and to reverse the

current color of the drawing surface.

•

The Font property determines the font used to write text in the form, using

the TextOut method of the canvas. A font has a Name, Size, Style, Color,

and so on.

TIP

Experienced Windows programmers should note that a Delphi canvas technically

represents a Windows device context. The methods of the TCanvas class are simi-

lar to the GDI functions of the Windows API. You can call extra GDI methods by

using the Handle property of the canvas, which is a handle of an HDC type.

Colors

Brushes, pens, and fonts (as well as forms and most other components) have a

Color property. However, to change the color of an element properly, using non-

standard colors (such as the color constants in Delphi), you should know how

Windows treats the color. In theory, Windows uses 24-bit RGB colors. This means

you can use 256 different values for each of the three basic colors (red, green, and

blue), obtaining 16 million different shades.

However, you or your users might have a video adapter that cannot display

such a variety of colors, although this is increasingly less frequent. In this case,

Windows either uses a technique called dithering, which basically consists of

Delphi 5 - Mastering Delphi 5 - Chapter 22 - Graphics.pdf

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