* THE OUTPUT PAGE

The largest part of the output page is occupied by a canvas and a legend.
In the legend GTKMathplot displays the captions of the objects drawn in 
the canvas; of course, no caption is written for objects which do not
have an associated caption (see section [[THE INPUT PAGE]] to know how to
associate a caption to a curve or surface).

If there are objects visualized in the canvas, this one shows on its top side:

- the coordinates of the point of the plane/space which has been put to
  its center, and

- the step of the grid, which is displayed if the "grid" check button
  is toggled on.

Besides the canvas and the legend, the output page contains several controllers
to change the aspect of the graphic, and many buttons.

The three adjusters on the left of the canvas, labeled with "Longitude:",
"Latitude:" and "Rotation angle:" respectively, can be used to change the point
of view of the observer. Three-dimensional curves and surfaces are drawn by 
GTKMathplot using a sort of axonometric projection. 
They are visualized on the screen as an observer would see them from very far
away using a telescope (a special one, which does not introduce special
optic effects nor distorts the shapes, but only magnifies them). 
The adjusters can be used to change the observer's position
while keeping fixed the distance between the observer and the observed shape;
thus, the observer's position is constrained to be on a sphere
with very large radius (ideally infinite) centered at the place
occupied by the shape.
You can move the position of the observer on this ideal sphere
by changing the values of his longitude and latitude.
In addition, you can change the rotation angle, which corresponds to rotate
the telescope of the observer around the axis of its tube.

In two-dimensional graphic mode the adjusters for longitude and latitude
are disabled, and you can change only the rotation angle. If you do it,
you will see your graphic rotate around the center of the canvas.

Mind that longitude and rotation angle can take any integer value between
0 and 360 (degrees), while the latitude may vary only between -90 and 90
(degrees). To change any of these values you have to click on the arrows
on the right side of the corresponding adjuster (Remark: if GTKMathplot has
been built with GTK+3.0 or higher, then the arrows are replaced by the 
buttons "+" and "-"). The up-arrow (or "+" button) increases the
value, the down-arrow (or "-" button) decreases it. The quantity of which the
value is in-/decreased depends on which mouse button you press: 
for the left button is 1, for the middle button is 15, while the right 
button sets the value directly to its maximum or minimum.

Below the adjusters you can see two scales with sliders.
The slider on the "Zoom" scale can be used to enlarge the visualized
shapes or to make them smaller. The default value of the "Zoom" is 100%.
You can increase it till to 1000% and decrease it till to 50%
by dragging the slider.

The slider on the "Smoke density:" scale is enabled only in three-dimensional
graphic mode. In three-dimensional mode GTKMathplot uses the so called smoke
effect to simulate a three-dimensional appearance and give you a feeling of 
the distance of the different parts of a shape from the observer's position
(which is your position, since you are the observer).
The smoke effect is a trick to let your eyes see three-dimensional curves
and surfaces even if GTKMathplot draws all shapes on a planar surface,
the one of your screen.
The trick consists in changing the color which the facets or segments forming
a shape are drawn with: the parts of a shape "nearer" to you are
drawn with a brilliant color, which color exactly depends on the
current settings, and as the distance from your eyes increases
the color drifts gradually toward a deep black.
The speed at which this gradual drift takes place is controlled by the
smoke density: a higher density means less brilliant colors, a lower
density involves a slower drift towards the black.
The default value for the smoke density (0.5) is adequate for the greatest part
of use cases, you will rarely need to change it.
By dragging the slider you can change the smoke density from a minimum value of
0 to a maximum value of 1.

To drag a slider you should click on it and move it while holding down the
mouse button. Alternatively, you can use the arrow keys after activating
the slider by navigating to it with the TAB (tabulator) key: left- and up-arrow
move the slider to the left and decrease the associated value, right- and
down-arrow move the slider to the right and increase the corresponding value.
If you want to move in bigger steps, hold the Control key down while pressing
an arrow key.

Under the sliders the Output page shows four check buttons, all contained
in the same frame. They look as small squares and each of them has a label
on its right side with the name of the associated option.
Whenever you click on one of them, the corresponding option is
toggled on/off.
The check buttons can be used to enable/disable the visualization of:

- x, y and (only in 3D-mode) z direction,

- grid lines (in two-dimensional graphic mode) or points
  (in three-dimensional mode),

- labelled tics along the x, y, and z direction,

- discontinuity points of curves and surfaces (this is an
  experimental feature, works pretty well only for curves and
  is available only in two-dimensional graphic mode).

By default the visualization of these optional elements is disabled.
Toggling a check button on(off) enables(disables, respectively) the 
visualization of the associated element.
In two-dimensional graphic mode x and y direction (axis) are always drawn
through the center of the canvas.

Under the frame with the check buttons you can see three buttons.
If you click on "Back to input mask" (keyboard shortcut Alt+B), you will be
moved back to the input page.

If you click on "Save as..." a file selector appears and you can choose
a file where to save the graphic currently shown in the canvas.
The file can have an arbitrary name, but its extension should always be either
".pdf", ".ps". ".eps", ".svg", or ".mgr", like in "ArbitraryName.mgr".
The chosen extension determines the format used to save the image to the file,
according to the following table:

          | extension | format                     |
          |-----------+----------------------------|
          | .pdf      |   PDF                      |
          | .ps       |   Postscript               |
          | .eps      |   Encapsulated Postscript  |
          | .svg      |   Standard Vector Graphic  |
          | .mgr      |   GTKMathplot Graphic      |

If the file name you choose corresponds to an already existent file, you will
be asked for confirmation before the file is overwritten with the image 
shown in the canvas.

If you click on the "Load object from file" button you get the possibility
to visualize the image previously stored in a file.
When the file selector appears, you should choose the file where the image
that you want to load has been saved. 
You can load an image either from a file containing data in GNUplot-compatible
format whose name ends with the extension ".gdat", or from a file
in GTKMathplot's own image format and whose name ends with the 
extension ".mgr". Other extensions or file formats are not accepted.

Remeber that files created by clicking on the "Evaluate & Save" button
of the input mask (see section [[THE INPUT PAGE]]) are always in GNUplot's
format.
To know more about the format of these files, see the section 
[[FORMAT OF GDAT FILES]].

GTKMathplot's own image files are only useful for debugging purposes, since
their format is quite cumbersone, and they cannot be easily parsed.

Mind that, after loading one or more images in the canvas, they remain
displayed as long as you do not move back to the input page and press
the button "Display/Update graphic". Whenever you ask for un update,
GTKMathplot redraws the contents of the canvas by considering only
the mathematical shapes defined in the input page. 

At the bottom of the output page a row of seven buttons is shown.
They can be used to move the shapes in the canvas along the directions x, y,
and (only in 3D-mode) z, or to reset their positions to the points
where the shapes were placed immediately after displaying or updating the 
graphic for the last time.
To reset the positions of the shapes you should click on the "Reset" button.

If you click on the "x+", "y+", or "z+" button, the objects drawn in the
canvas are moved simultaneously in the direction of increasing x, y, or z
coordinates, respectively.
If you click on the "x-", "y-", or "z-" button, the shapes in the
canvas are moved simultaneously in the direction of decreasing x, y, or z,
respectively.
How far the shapes are moved depends on which button of the mouse you press
while clicking. If you click with the left mouse button, then the
step of the movement is equal to the grid step written on the top of the 
canvas. If you click with the right mouse button, the step of the
movement will be equal to 1/10 of the grid step.

Mind that in two-dimensional graphic mode the buttons for the z direction
are disabled, and

- clicking on "x+" ("x-") moves the shape to the left (right, respectively),

- clicking on "y+" ("y-") moves the shape up (down, respectively) in the
  canvas.

** SHORTCUTS FOR THE OUTPUT PAGE

- Alt+l : Open file selector to load images from a file
- Alt+s : Open file selector to save the current contents of the canvas to a file
- Alt+b : Switch back to the Input page
