DRAWING in Java

Creation QR in Java DRAWING

of one point on the circle itself. The only way to find out which of these specifications a drawing package uses, is to read its documentation. In order to deal with all these ways of representing objects on a canvas, it helps if you are comfortable with geometric algebra, and can easily translate any of the specifications into something with which you are more comfortable. In this chapter we ll look at how the various modules available for Perl deal with drawing primitives, what some of these primitives are, and how they can be used.

GD provides a range of drawing primitives and some auxiliary methods, such as color allocation, color flood fill, and methods to set a brush. These methods are listed in table 4.1, and the text drawing methods are listed later in this chapter, in table 4.3 on page 45.

Table 4.1 The drawing primitives for the GD module which are available to the user of the module to create images. All of these methods should be called on a GD::Image object. setPixel(x,y,color) line(x1,y1,x2,y2,color) dashedLine(x1,y1,x2,y2,color) rectangle(x1,y1,x2,y2,color) filledRectangle(x1,y1,x2,y2,color) polygon(poly,color) filledPolygon(poly,color) arc(cx,cy,w,h,st,end,color) fill(x,y,color) fillToBorder(x,y,bcolor,color) setBrush(brush) setStyle(color-list) set the color of the pixel at the specified coordinates draw a solid line between the specified points draw a dashed line between the specified points draw a rectangle with the specified corners draw a filled rectangle with the specified corners draw the polygon specified by poly, which is a polygon object created with GD::Polygon::new() draw the filled polygon specified by poly draw an arc with the specified center, width and height, and start and end angle flood-fill all pixels with the same color around the specified point flood-fill all pixels around the specified point, until a pixel with the specified bcolor is encountered Set the brush to be used to the specified brush, which is another GD::Image object Set the line style to the specified color list. Each element of the list represents a single pixel on the line to be drawn.

Most drawing methods accept a color argument, which is either the index of a color previously allocated with one of the color allocation methods, or one of gdBrushed, gdStyled or gdStyledBrush. These three will cause the object to be drawn with the image s current brush, line style, or both.

Unfortunately, there is no filled form of the arc() drawing primitive, which can cause some hardship when trying to create filled partial circles or ellipses. There also is no variant of the arc primitive which allows you to specify a start and end point of the arc. Thus, it is almost impossible to reliably create a pie slice (or a pizza slice) that can be flood-filled. Because of little rounding errors it is possible that there are gaps between the arc that forms the outside bit of the slice, and the lines that form the wedge. When subsequently one of the fill methods is called, these little gaps will cause the flood-fill to leak out into the rest of your drawing. If there were an arc command that allowed one to specify start and end points, it would be possible to reliably close these gaps.

GD has its coordinate origin defined in the top left-hand corner of the canvas, at coordinate point (0,0). This means that all pixel coordinates are offset at 0, and the coordinates of the center point of a 100 by 100 pixel image are at (49,49). In order to use colors in GD you first need to allocate them for your object. The total number of colors in a GD::Image object is limited to 256, which for most drawing purposes is fine (see also section 12.1, GD and pixels, on page 211). You will find that you normally don t use more than about 10 colors anyway.