Graphing trigonometric functions - php

Question

I've come across something that I'd like to accomplish without using any frameworks or other graphing tools that are out there on the internet, which is graphing trigonometric functions using only PHP and if needed, SQL. I'm aware of the GD library, but none of the functions were helpful. I wrote a small script, although it doesn't really work either. My goal is to do the following:
Allow for trig function name, starting value and end value to be added as parameters.
Check whether a function is cos, tan or sin.
Loop through all values given starting and end values in degrees, and convert to radians.
"Add" all values to an array, if needed, and graph the function given the points.
After looping through all the values of the function, what needs to be done to graph the function? Do the values have to be in a seperate array? What functions need to be used for graphing?
<?php
header("Content-type: image/png");
function graphFunction($function, $startDegree, $endDegree)
{
$functionList = array('cos', 'sin', 'tan');
if (strtolower($function) == 'cos')
{
$cosValues = array();
for ($c = $startDegree; $c < $endDegree; $c++)
{
array_push($cosValues, cos(deg2rad($c)));
$graph = imagecreatetruecolor(500,250);
$col_poly = imagecolorallocate($graph, 255, 255, 255);
imagepolygon($graph, [the cosine values] , 34, $col_poly);
imagepng($graph);
imagedestroy($graph);
}
}
}
echo graphFunction('cos', 0, 360);
?>
This is supposed to be a sample function, so no need to be criticizing the useless control structure as there are ways to store many things in a database, whatsoever. I hope for some feedback, and hopefully it is possible with PHP.

Answer 1

You can do this with PHP. I just tried with the following inside your if statement, and it seems to work.
$height = 250;
$offset = $height/2;
$graph = imagecreatetruecolor(500, $height);
$col_poly = imagecolorallocate($graph, 255, 255, 255);
for ($c = $startDegree; $c < $endDegree; $c++)
{
$this_x = $c;
$next_x = $this_x + 1;
$this_y = cos(deg2rad($this_x)) * $offset + $offset;
$next_y = cos(deg2rad($next_x)) * $offset + $offset;
imageline($graph, $this_x, $this_y, $next_x, $next_y, $col_poly);
}
imagepng($graph);
imagedestroy($graph);
So what I did was define a graph height, because the y coordinates of the points in the graph need to be set to they fit inside. Then inside the for loop I just take the current degree value as my x coordinate, calculate the y coordinate for it, then the same for the next degree value, and then I draw a line between those two points.
It's a bit cludgy, so I'm sure you'll wan to clean it up a bit. Also, you need to think about the width of your graph as well (what I've posted here will stop at 500 px wide, so if you want to graph a wider range, or if you want to plot 300 - 600 degrees it will just start halfway into your graphing area.
You also defined your image and colours inside the for loop, which I changed so you're not recreating an image resource each step and losing all previous data.
Anyway, it was just an example function, right? But using your degree values as x coordinates (adjusted to your graphing area sise), computing y coordinates based on the chosen function, the degree values and your graphing area size and then drawing lines between the points should work.