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From version 1.2 JpGraph supports drawing of anti-aliased lines. There
are a few caveats in order to use this which is discussed in this
section.
Sidebar Note that anti-alising will not be
used for either horizontal, vertical or 45 degree lines since they are
by their nature are sampled at adequate rate.
Anti-aliased lines are enabled by calling the method
SetAntiAliasing() in the Image
class in the script where you want to use anti-aliasing.
The anti-aliasing for lines works by "smoothing" out the edges on
the line by using a progressive scale of colors interpolated between
the background color and the line color.
Sidenote: The algorithm used for
anti-aliasing of lines is quite simple. It would be possible to achieve
even better result by doing some real 2D signal processing. However,
doing real time 2D signal processing on a HTTP server would be madness
so I deliberately kept it simple. To achieve best visual result always
use a dark line color on a light background.
An example will show that this, quite simple algorithm, gives a
reasonable good result. The figures below shows a radar plot with and
without anti-aliasing.
Figure 1: Spiderplot without anti-aliasing
[src]
|
Figure 2: Spiderplot with anti-aliasing
[src]
|
One thing you need to keep in mind when deciding to use
anti-aliasing is that it could have potentially a dramatic effect on
the time it takes to generate the image. Line drawing with
anti-aliasing turned on is roughly 8 times slower than the normal line
drawing so treat this feature wisely.
Furthermore there are a couple of "gotchas" you should be aware of
when using anti-aliasing.
- Anti-aliased lines uses up more of the available color-palette. The
exact number of colors used is dependent on the line-angle, a near
horizontal or near vertical line uses more colors (number of lines with
different angles uses more colors). Hence it might not be possible to
use anti-aliasing with color-gradient fill since the number of
available colors in the palette might not be enough. A normal palette
can keep around 256 colors. This means that you are advised to use a
truecolor image when using anti-aliasing.
- Anti-aliasing does not work very well together with background
images since it assumes a the same solid color on each side of the
line. Doing a more advanced anti-aliasing algorithm would simple take
to much processing power.
- Anti-aliased lines will ignore the line width specified. They will
always have a width of roughly 1.
JpGraph provide the possibility for you to rotate the generated graph
an arbitrary angle. This will only affect the actual graph (axis, axis
titles, labels and so on) and not fixed elements on the graph like
title or footer.
Rotation is probably most used to rotate a graph 90 degrees, for
example a bar graph to get the effect of horizontal bars.
Performance note: Adding a rotation
transformation will make the graph generation slightly slower since
each point of the graph as to go through a transformation step before
being stroked on to the image. JpGraph optimises this by using a
pre-calculated transformation matrice and also optimises the special
case 90 degrees.
By default the center of the rotation will be the center of the plot
area, which may or may not coincide with the center of the entire
image.
To control the rotation you use the two methods
For example
$graph
->image->SetAngle(
45);
There is actually a third method that you could use, adding a
translation to the graph after the rotation. Since
this probably a very little used method we don't discuss it further but
refer the reader to the class reference instead
Graph:image::SetTranslation()
When you rotate an image you should be aware of that the individual
labels on the axis are not rotated. The design decision behind this is
a) Bit mapped font can't be rotated
b) Maintain readability
Please remember that you may still rotate the labels by calling the Axis::SetLabelAngle()
method.
Since the anchor point for labels is by default the optimum for
graph at 0 degree you might want to adjust the anchor point and
alignment for the labels on the axis to get a better visual appearance
on you rotated graph. This is accomplished by the method
Axis::SetLabelAlign() For a detailed discussion on how to do this
please see the section on horizontal bar graphs, (
Working with bar plots )
The table below shows some examples on different kinds of rotation
to give you an idea of how changing the angle and rotation center may
be used to generate different effects. The top left graph is the
original image. The point of rotation has been marked with a red-cross
in each of the images.
Figure 3: Original image
[src]
|
Figure 4: Rotated 45 degrees around center of plot
area [src]
|
Figure 5: Rotated 90 degrees around center of plot
area [src]
|
Figure 6: Rotated 45 degrees around center of the
image [src]
|
Figure 7: Rotated 90 degrees around center of the
image [src]
|
Figure 8: Rotated -30 degrees around the lower
left point of the plot area [src]
|
As you can see from the images above if you rotate about any other
point than the center of the plot area the plot can be placed outside
the image after rotation.
Since the rotation, by design, only affects the plot area it is
often most effective to use when the color of the margin is the same as
the background color.
It is often desirable to have a background image look a little bit
"washed" out so it doesn't take the concentration away from the actual
graph. There are basically two ways of accomplish this
- Prepare the image with an external images editor to adjust the
level of brightnes and contrasty to a desirable level
- Use JpGraph:s built int adjustment for contrast, brightness and
color saturation.
To adjust the background image call The levels for both brightness and
constrast are real numbers in the range [-1, 1] You can choose to
adjust for example just the background image or you might also choose
to adjust the whole image. To change the background image just use the
method
Graph::AdjBackgroundImage() to specify a suitable value. Let's show
some example on what we can do with this. The following example have
been generated by using the small utility "adjimg.php" which you can
find in the "utils/" directory.
Brightness=0, contrast=0, saturation = -1 (Original image) |
Brightness=0, contrast=0, saturation = -1 (Black White image) |
Brightness=0.3, contrast=-0.3, saturation=0 |
Brightness=0.4, contrast=-0.7, saturation=0 |
Brightness=0.4, contrast=-0.7, saturation=-1 |
Brightness=0, contrast=0, saturation=1 |
During development and optimization it can be very handy to have the
actual time it took to generate the image as a footnote. The following
example shows the usage of this feature
Figure 9: Timing of a graph
[src]
To enable this feature you can proceed in two ways.
- You can either set the global define BRAND_TIMIING (in jpgraph.php)
to true. This will add the timing string to all graphs generated.
- .. or you can enable it for a specific graph by setting the global
variable $gJpgBrandTiming as in
$gJpgBrandTiming=true
;
in the beginning of the script.
If you like you might also change the way the timing is formatted by
setting the string defined by BRAND_TIMING_FORMAT (in jpgraph.php).
This string represents a standard printf() format string.
Sidenote: JpGraph contains a utility class
called JpgTimer which you can use yourself should you need ms timing of
part of your own code. The API is really simple. The class supports
multiple running timers and you start a timer simply by calling the
Push() method. This will start a new timer and put it on the top of the
timer stack. To stop the timer, pop it from the stack and return the
timing value simply call Pop().
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