Web Programming with html5, css, and JavaScript

FIGUre 12.23G
shows the rest of 
solarAngles.js
. The 
runClock
function executes 
when a user clicks the web page’s 
Run Clock
button. It starts by initializing the global 
timer
variable with a 
handle,
which refers to a 
handler
method. It does this by invoking a predefined 
window
method called 
setInterval
, whose first argument is the name of the handler method 
the browser is to call at the ends of successive time intervals, and whose second argument is the 
desired length of those time intervals in milliseconds. The handler method is usually a custom 
JavaScript function. In the present case, it’s the 
updateDisplay
function also in Figure 12.23G. 
621
12.12 Case Study: Solar Shadowing Dynamics

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FIGUre 12.23F 
JavaScript file for Solar Shadowing web page
function getObscuration(x0, y0) {
var latitude; // earth latitude
var projLength; // north-south horizontal projection
var width; // east-west width of panel array
var tanShadeAngle; // tan(shade angle)
var shadeLength; // upward in panel plane
var offset; // east-west shadow offset
var obscuredWidth; // east-west inter-panel shadow extent 
var obscuration = 0; // fractional inter-panel shadowing
latitude = form.elements["latitude"].value * Math.PI/180;
projLength = image.height * Math.cos(slope);
width = image.width;
context.globalAlpha = 0.4; // shadow opacity
context.fillStyle = "black";
// Sun behind panels
if (cosIncidenceAngle <= 0) {
// cover bottom edge but keep top edge white
if (slope > 0) {y0 = y0 + 2 - projLength;}
context.fillRect(x0, y0 - 1, width, projLength);
} // end if
// Inter-panel shading
else if (Math.cos(azimuth) * (y0 - 0.5 * canvas.height) > 0 &&
latitude * slope > 0) {
tanShadeAngle = Math.tan(altitude) / Math.cos(azimuth - Math.PI);
shadeLength = length 
- spacing / (Math.cos(slope) + Math.sin(slope) / tanShadeAngle);
if (shadeLength < 0) {shadeLength = 0;} // no obscuration
projLength = shadeLength * Math.cos(slope);
offset = Math.abs(Math.tan(azimuth) *
(spacing + projLength - length * Math.cos(slope))); 
if (slope > 0) {y0 += 1;} // cover image bottom
else {projLength = - projLength;}
if (Math.sin(azimuth) < 0) {
context.fillRect(x0 + offset, y0, width - offset, - projLength);}
else {context.fillRect(x0, y0, width - offset, - projLength);}
obscuration = (shadeLength * (width - offset)) / (length * width);
if (obscuration < 0) {obscuration = 0;}
} // end else if
context.globalAlpha = 1.0;
return obscuration;
} // end getObscuration