Guide on Testing a Shape with Multiple Paths
A complex shape made up of 2 paths: Path1= 4 thin red lines, Path2= 2 thick blue lines.
Understanding Paths for Accurate Hit-testing Results
The use of the beginPath method can impact how path recognition functions, especially when using the isPointInPath method.
You have the flexibility to define multiple paths within your design, but keep in mind that only the last defined path will be tested by the isPointInPath function.
A path comprises a series of path commands structured as follows:
- A path commences with the
beginPath command
- The path specifies its shape using commands such as
moveTo, lineTo, and so forth.
- To conclude a path's definition, another
beginPath command should be employed.
Therefore, in your current code snippet, only the final moveTo+lineTo segment will undergo testing.
context.beginPath();
context.moveTo(this.clickedX+this.width, this.clickedY-1);
context.lineTo(this.clickedX+this.width, this.clickedY+TAB_ABSTAND*(TAB_SAITENZAHL-1)+1);
In scenarios involving a single line with a substantial stroke (e.g., context.lineWidth=10), the isPointInPath method may not return true for straight lines devoid of curves.
Mathematically speaking, linear structures occupy no area, rendering it impractical to ascertain if any point lies "within" a solitary line.
With most browsers (excluding IE/Edge), you can leverage the new isPointInStroke technique to verify a point's location within a lone line. For consistent cross-browser compatibility, converting a single line into a path allows for effective hit-testing of points within that modified path.
The closePath operation connects the concluding endpoint of a previous line to the starting point of an initial line without breaking the ongoing path sequence, ensuring that subsequent strokes incorporate adjustments from lineTo and moveTo methods preceding the closePath command.
Conversely, closePath denotes an inefficient term since it fails to truly terminate (or seal) a path structure. It does not act as the "closing brace" counterpart to beginPath's "opening brace." Instead, it essentially forms a direct line from the present path position back to the initial point in the path.
When attempting to draw extended lines without proceeding through beginPath(), the process might result in related path components getting stroked unintentionally.
Each path is restricted to a singular style application. Therefore, when incorporating varied lineWidth specifications amidst a set of path directives, the concluding lineWidth value takes precedence, impacting the overall stroke appearance across the entire path.
moveTo(x,y) actively relocates the drawing tool, creating potential path configurations that are recognizable by the isPointInPath method. This effect applies particularly to varieties extending beyond mere linear constructs.
moveTo signifies a translation akin to shifting a pen to fresh coordinates on a piece of paper without terminating the existing course of path actions.
For example: If you initiate a path with beginPath and proceed with delineating 3 distinctive triangles via moveTo separations, all 3 triangles become integrated within the path realm and subjected to isPointInPath evaluations.
The utilization of the fill() approach serves as a valuable tool for visually depicting various path formations.
The behavior triggered by .fill and
.stroke</code requests entails visually rendering the prevailing path scheme onto the canvas display. Such directives do not mark the conclusion of the path itself — solely another <code>beginPath
invocation concludes an active path series. Thus, situations where interdisciplinary activities occur, like outlining 2 edges of a triangle and subsequently filling in the third edge prior to repeat invoking could lead to asymmetric appearances between earlier strokes and the ultimate one.
Key Information: You have the liberty to plan and assess distinct paths without executing stroke or fill directives, effectively enabling isolated redefinitions + isPointInPath assessments on each path sans necessitating additional redraws on the canvas medium. Moreover, it remains feasible to create a unified form comprised of varied path segments then engage in inclusive path evaluation processes using isPointInPath.
To discern which shapes (identified as paths here) register interaction during a mousedown event, consider redefining every multi-path element and apply trials via
context.isPointInPath(mouseX,mouseY)
.
For literal drawing tasks, distinct styles must be assigned based on line thickness differentials, mandating individual drawing steps for each path due to sole styling allowance per path.
Outlined below is sample code along with a Demo for reference:
var canvas=document.getElementById("canvas");
var ctx=canvas.getContext("2d");
var cw=canvas.width;
var ch=canvas.height;
function reOffset(){
var BB=canvas.getBoundingClientRect();
offsetX=BB.left;
offsetY=BB.top;
}
var offsetX,offsetY;
reOffset();
window.onscroll=function(e){ reOffset(); }
window.onresize=function(e){ reOffset(); }
var isDown=false;
var startX,startY;
var shapes=[];
// Define path1 comprising specified points
var path1=[
{x:150,y:100},
{x:50,y:100},
{x:25,y:75},
{x:50,y:50},
{x:150,y:50}
];
path1.linewidth=1;
path1.strokestyle='red';
// Define path2 with specified coordinates
var path2=[
{x:150,y:50},
{x:225,y:75},
{x:150,y:100}
];
path2.linewidth=5;
path2.strokestyle='blue';
// Compose shape1 composed of path1 and path2
var shape1=[path1,path2];
shape1.fill='green';
//
shapes.push(shape1);
// Display both sections of the path formation on canvas
draw(path1);
draw(path2);
$("#canvas").mousedown(function(e){handleMouseDown(e);});
function define(shape){
ctx.beginPath();
for(j=0;j<shape.length;j++){
var p=shape[j];
ctx.moveTo(p[0].x,p[0].y);
for(var i=1;i<p.length;i++){
ctx.lineTo(p[i].x,p[i].y);
}
}
}
// Implementation for visualizing path construction
function draw(path){
ctx.beginPath();
ctx.moveTo(path[0].x,path[0].y);
for(var i=1;i<path.length;i++){
ctx.lineTo(path[i].x,path[i].y);
}
ctx.lineWidth=path.linewidth;
ctx.strokeStyle=path.strokestyle;
ctx.stroke();
}
// Functionality to place markers at specific positions
function dot(x,y,fill){
ctx.beginPath();
ctx.arc(x,y,2,0,Math.PI*2);
ctx.closePath();
ctx.fillStyle=fill;
ctx.fill();
}
function handleMouseDown(e){
// Signifying exclusive handling of this event
e.preventDefault();
e.stopPropagation();
// Reveal mouse coordinates
var mx=parseInt(e.clientX-offsetX);
var my=parseInt(e.clientY-offsetY);
//
var dotcolor='red';
for(var i=0;i<shapes.length;i++){
define(shapes[i]);
if(ctx.isPointInPath(mx,my)){dotcolor=shapes[i].fill;}
}
dot(mx,my,dotcolor);
}
body{ background-color: ivory; }
#canvas{border:1px solid red; }
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.9.1/jquery.min.js"></script>
<h4>Click inside and outside the multi-path shape<br>Inside clicks become green dots.</h4>
<canvas id="canvas" width=300 height=300></canvas>