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Chapter 9Graphics Programming
By Felipe Monteiro de Carvalho
When we speak about graphics, we could be referring to two
separate topics.On the one hand we may be talking about the
manipulation of pictures or images how to handle existing graphic
data. On the other hand, we may be thinking
about how to draw on a canvas and display a picture. These are
related andoverlapping topics.Lazarus offers very powerful tools
for both these activities. The control canload graphics in several
formats from files and resources; and various visualcontrols
together with the TCanvas class provide a drawing surface, which
can bemanipulated and drawn upon.
LCL
TImage
canvas canvas
TBitmapCanvas
TPoint TRect TSize
9.1 The CanvasThe is the basis for all drawing operations. The
is a property of visual
components and it is derived from graphics classes such as .
Being a classproperty has no memory of its own. That is why an
error occurs if you try to usea canvas before you have instantiated
a visual control for it. The takes care of thenecessary
preparations for drawing visual components on a form, and for
displayingvisual components onscreen.Drawing on a canvas is always
done using X,Y co-ordinates. The co-ordinate system'sorigin is
always at the top left corner of the working area. The window's
title bar isoutside this area, so it is not affected. Drawing is
done either using separate X and Yinteger co-ordinates, or using
co-ordinates which are grouped together as records, eitheras a (a
record linking an X and a Y co-ordinate) or a . Some graphic
methodsdo not work with absolute co-ordinates, but with a record
instead. Such a recordonly defines the width and height, but not
the target position.
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9.1 The Canvas
= record
: ;
: ;
end;
= ;
= record
case of
0: ( , , , : );
1: ( , : );
end;
= ;
= record
: ;
: ;
end;= ;
procedure . ( : ; : ;
: ; , : );begin
. ( , );
. (100, 100);
end;
TPoint
X LongInt
Y LongInt
PPoint ^TPoint
TRect
Integer
Left Top Right Bottom LongInt
TopLeft BottomRight TPoint
PRect ^TRect
TSize
cx LongInt
cy Longint
PSize ^TSize
TForm1 FormMouseDown Sender TObject Button TMouseButton
Shift TShiftState X Y integer
Canvas MoveTo x y
Canvas LineTo
The real declaration is more complex. It ensures that the type
declarations are identical inthe units , and . However, the result
shown above is how developerssee it (and should see it). A little
example will show how simple the philosophy behindLazarus drawing
operations is. We create a new Lazarus application and extend
the
event of the window with just two lines:
After a mouse press on the form X and Y will contain the actual
X and Y co-ordinates.The drawing tool is moved to that location and
then a line is drawn to thepre-programmed location (100, 100) in
the window. This drawing operation is done onthe window canvas.
This will always happen, unless a different control's canvas
isspecified. This work is volatile. If the window is updated (for
instance when its sizechanges) the lines drawn earlier will
disappear.
The canvas is capable of many more operations than this simple
one.The most important methods and properties in the following
definition are brieflyexplained in Table 9.1. We will see more in
the examples that follow later.
As the work done on the form's canvas is not retained, the
graphic data will have to bepainted again after every update of the
control. For the graphic to persist, therefore,
we will have to keep a list ourselves of the completed drawing
operations in a buffer.
Classes Types Windows
FormMouseDown
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