我在canvas元素上工作,并且我在旋转画布元素处停留。
使用我的项目用户单击画布上的任何位置并通过fillText生成文本
到目前为止它还可以。但是当用户尝试基本旋转它时它失败了,因为我坚持翻译和特定坐标的翻译问题。我在互联网上研究所有的例子都使用画布中心。我不能使用它,因为我的画布大小2406 * 2406和生成的文本必须在用户点击的坐标下维护。我希望我描述得很好,因为英语不是我的主要语言。 感谢您的未来帮助..
答案 0 :(得分:1)
想象一下画布是页面上的一张网格纸。该网格纸称为变换矩阵。左上角是原点(x:0,y:0),x向右,y向下到页面。您绘制的所有内容都与该点及其x和y相关。因此,200,200处的绘图文本右边200像素,距原点200左右(左上角)。
如果您使用翻译,旋转,缩放,设置变换或变换,则可以在页面上移动网格纸张。比方说翻译200,200。您已将原点向右移动200像素,向下移动200像素。现在,如果您在200,200处绘制文本,它仍然相对于网格纸的原点,现在不在左上角。您的文字最后距离左上角400 x 400像素。如果您想在翻译之前的同一位置绘制文本,则必须更改为坐标以考虑翻译。
旋转会改变网格的方向。顺时针旋转Math.PI / 2(90度)会导致页面侧面的网格。沿x方向移动不再是从左到右,而是从上到下。
规模扩大了合同的网格纸。大于1的比例会增加每个网格的大小,小于1的比例会减小每个网格的大小。
使用平移,缩放和旋转的组合,您可以将网格纸定位在页面的任何位置。当您绘制文本或任何内容时,您总是将其绘制为与网格对齐。
举例说明如何绘制缩放的旋转翻译文本。请花点时间了解发生了什么。如果您有任何疑问,请仔细阅读。 (手指交叉,因为我希望这可行,因为这是我第一次尝试使用stackoverflow代码片段的东西)
// use matix
var useMatrix = false; // how to create the transformation
// mouse stuff
var mouse = {
x:0,
y:0,
};
function mouseMove(event){ // mouse event handler with firefox mod
mouse.x = event.offsetX;
mouse.y = event.offsetY;
if(mouse.x === undefined){ // if firefox
mouse.x = event.clientX;
mouse.y = event.clientY;
}
}
var ctx;
if(ctx === undefined){ // create the canvas if it does not exist
var can = document.getElementById("canvasID");
if(can !== null){
ctx = can.getContext("2d"); // get the context
can.addEventListener('mousemove',mouseMove); // add the mouse
}
}
// some stuff to animate some text
var angle = 0; //
var angleDelta = 0.1; // rate of change for angle
var scaleX = 1; // to hold scale x and y
var scaleY = 1;
var counter = 0; // counter used to change scale over time
var counterDelta = 0.1; // rate of scale change
var scaleFunction = function(){ // function to do scale change
scaleX = Math.sin(counter)*0.4 + 1;
scaleX = Math.cos(counter)*0.4 + 1;
counter += counterDelta;
}
// The drawing function will call the drawing callback after it has
// set the coordinate system to the desired position,rotation and scale.
// set coordinates to
// translate x,y
// rotate angle in radians
// 0 is no rotation
// Math.PI/2 is clockwise 90 deg
// -Math.PI/2 is antiClockwise 90 deg
// Math.PI is 180 deg
// scale sx along x
// sy along y
// ctx the context to draw to
// drawing the function that will draw
function drawAt(ctx,drawing,x,y,sx,sy,angle){
if(useMatrix){ // create the transform by hand
var xdx = Math.cos(angle); // get the direction of x along the page
var xdy = Math.sin(angle);
ctx.setTransform( // completely replace existing matrix
xdx * sx, // set the x direction and scale it by sx
xdy * sx,
-xdy * sy, // set the y direction @ 90deg of x and scale it by sy
xdx * sy,
x,y // set the origin at x and y
);
} else { // create the matrix by mutiplying existing matrix with translation, rotation and scale matricies.
// the order of these are important
ctx.translate(x,y); // multiply existing matrix with this translation
ctx.rotate(angle); // multiply the modified matrix with this rotation
ctx.scale(sx,sy); // multiply the modified matrix with this scale
}
drawing(ctx); // call draw
// restor the transform to default (identity matrix)
// reset x direction is vector (1,0) from left to right
// y (0,1) from top to bottom
// origin is 0,0 at the top left of page.
ctx.setTransform(1,0,0,1,0,0); // reset the transform to top left
}
// the function to draw. It does not need to now about where it is being draw.
function drawHello(ctx){ // say hello
ctx.font = "20px verdana"; // all drawing is relative to the origin
ctx.textAlign = "center";
ctx.textBaseline = "middle";
ctx.fillText("Hello planet Earth!",0,-22); // draw above in the y direct -22 pixels
ctx.fillText("This is an example to help",0,0); // draw at origin
ctx.fillText("understand the coordinate system",0,22); // next line is 22 pixels below the top
ctx.font = "10px verdana"; // change font size
ctx.fillStyle = "white"; // and colour
ctx.fillText("Move the mouse over the canvas.",0,44);
}
// clear screen update animation values, and draw the text and what not.
function update(){ // function to update the canvas
ctx.clearRect(0,0,ctx.canvas.width,ctx.canvas.height); // clear the canvas
angle += angleDelta; // change the angle
scaleFunction(); // set the scales
useMatrix = ! useMatrix; // alternate transformation creation method.
ctx.fillStyle = "red"; // the red center
// first draw at the center of the canvas
drawAt(ctx,drawHello,ctx.canvas.width/2,ctx.canvas.height/2,1,1,0);
// now draw rotating ans scaling around the mouse
ctx.fillStyle = "black"; // black for the mouse
drawAt(ctx,drawHello,mouse.x,mouse.y,scaleX,scaleY,angle);
// set time out for the next update.
setTimeout(update,50); // refresh twenty times a second.
}
// only start if there is a canvas context to draw to.
if(ctx !== undefined){ // make sure there is something to draw to
update(); // start rendering;
}
.sofCanvas {
width:400px;
height:250px;
border: #000 solid;
background: #A4A4D1;
}
<canvas id="canvasID" class="sofCanvas" width=400 height=250>time to update your browser</canvas>