Canvas. How to get background pixels before drawing

Question

I created a small plug-in for my map application. This plug-in adds text labels to geometric features. It looks like so:

On the screen above you can see a map, a horizontal linestring and a text label. I created this label by using canvas, canvas.getContext("2d") and a bunch of standard functions like ctx.strokeText, ctx.fillText etc. The problem I face now is that the linestring on the screen is interactive or moveable and I want my label to move as well. I'm not asking about the exact solution to my problem. What I'm interested in is just how to get backround pixels (right below my text label), so that I could restore them before I "move" or redraw the label at a new place. If you can provide a teeny-weeny example where you have some background and then draw some object and then "remove" it, it will be great.

Answer 1

You probably want to use context.getImageData and context.putImageData

Assuming your canvas has the id "myCanvas", calling doDraw() will cause a black rectangle to blink on a complex background.

First, the background is drawn in doDraw(). Then, the background that is to be covered by the rectangle is captured in drawRectangle() and saved in the variable "imageData". Then the rectangle is drawn over the background. Then, 1 second later, eraseRectangle() is called, and the background is replaced by a call to putImageData().

In this fiddle:

https://jsfiddle.net/f3Luxcoc/

Here's the javascript:

//coordinates of rectangle
var xp = 20;
var yp = 20;
var wp = 80;
var hp = 80;

//saved background image
var imageData = null;

function doDraw() {

 var can = document.getElementById("myCanvas");
 can.width = 500;
 can.height = 500;
 var context = can.getContext("2d");

 //draw background contents  

 var image = getImage();
 context.drawImage(image, 0, 0);
 context.drawImage(image, 100, 0);
 context.drawImage(image, 0, 100);
 context.drawImage(image, 100, 100);

 drawRectangle();
}

function drawRectangle() {    

 var can = document.getElementById("myCanvas");
 var context = can.getContext("2d");

 //capture background 
 imageData = context.getImageData(xp, yp, wp, hp);

 //draw Rectangle
 context.rect(xp, yp, wp, hp);
 context.fill();

 setTimeout(function() {
  eraseRectangle();
 }, 1000);
}

function eraseRectangle() {
  var can = document.getElementById("myCanvas");
  var context = can.getContext("2d");

  context.putImageData(imageData, xp, yp);

  setTimeout(function() {
    drawRectangle();
  }, 1000);
}

doDraw();

function getImage() {
var image1 = new Image(237, 110);
  image1.src =     "data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAO0AAABuCAMAAAD8t2TLAAAAGXRFWHRTb2Z0d2FyZQBBZG9iZSBJbWFnZVJlYWR5ccllPAAAAC1QTFRF////LVcpkauOZYdhrsStx9jF1uTV+//66PPn8vzy4uji7vft4O3f+P/38fPwC7dd6gAACdxJREFUeNrsXIti2yoMNYiHiSH//7kXCXD8AmTaddudta1zEht0QOhIgnSaHnnkkUceeeSRv1vszzRlv7OjL2hoUZEwTSHUNHIsoLYJ1oYBvE2tjnfu1Nw85DbNWCVAxysvgS7OgxEcR8vglfINtDJ1cxNuRaur7t1OTUfoSPtNM1GLKHqaBUk4aakBP+73plBC9XONrbP03opJWs3dG1FLoT6vVeqMtFeTBZFQkhYCbEILFS37es2Etq6W4up99dTSXUewmyvrduj8RFMaYauMNtD/8ro/sN+FdhlB2x3sEBJas5ujFZ2c/O61CGl0TKW/8E1oYfolaO0idpOVp3qdy5AuVrRzuqGy3hgG6H4J2jJHXWcmxNYGk6FGS07rXtrkp4plRzj6utklW353eA3DSzH0PnsflitXeyertiBV5FfioNyeJDTB1dSUthc4xL+z0q6n90A8gEzCoVxZYJSXH5BIsvgZtRN6w6tyBFJFq2kAeyFCmIbEah7lhn2AswEp87wrIQ0n2lLN/nSxm6+GCNf+j70GYoBjdvFMcryFZMlxM+DmCKRNdr4TIuhBtIHr35boOPTRUxxpSH/VM2a0S8eDqEG0qXnorwONftKeu1V5jhUPrWpqa3uD/0W0+fl+PHWIXfPcFkvOEQUXbe2+atD5PWhLKD/z0No9C5CbTsFyqMRPl5Zcd7oOoAnmS+vW2uwk+4Pi9JED14kmcjWy75MDeSk511gP/X5YTIsSk08eqRuEKRBx2v6gBNtMsB2PA4MP1NQsL1nEOmcWojq8pxcjzHcn1kCb3A7pc8u+OByI3UkypmuWoUULsERzx/lvxgjNG+rUxwlcdWetKF60n5MHX0lPs4sSnhRrBFTZr99Dmxtn5Imq4wfVjWZKsn+R+eV0wyX/3uPkm64qN86ILDhouc1AnVML9yf/Dp10+yYN5TH6YbSFmutoc22klvaXdPsu6ep2RPBx1r11q3l9q22yr2oKSZsXtmun2wMhhsFaol3ZLhwSzfmjRtN1AystIQpAlwuXaS727xflyaEAkrutZVGCF8pcPJ2qpZQ4+pDpbKKCIsxEoimhJKJshR8Bb2mXpIN3HhuxmXftoQUfiko4B1ExexUPJb6WfgAsWiGFUhikCMC4dy2eUrKwKRarVnElKqDmdkmawMRRoQDiSOHUY+zQKDI3lw3ggjblPJ7Ny7QElm1Oowo7ldqTSJW2eh/pcTAdKzaTkwAg3dkV6I0/1kUdfTfO5mV8mRYyWrCLONSIXXOlWAcdCiyZngGUnCDC8YbsoTbq3MuhmLXisCubFspb0eZ6oqy3M4bWnlhOHdS5lx93fVQJ49QW3nLY7XDFBDpR6E2026WhPkHjVp3vRZssB5+GTXbuM/V90lfRqfPrDinoHdr5XEnXG5tC71Sphahh8ilUnVyfhTU7TxZrwiZ9LXlllYCa+W0hSU1eCncr02u/K+KWooBave91ukWlzoFVixjpWTfbizT9k76W+ngr/218PqWkViZXkfPTFU0u3tKo2oCDHDTqYyuuhtoYSOd92mQO3pi5nb42ybyX/8aJRJK0h/zUfopvngwpZvPExbPWmt4IZrmg3LRFcRdu5ECgcM2g9PJJM5z/Zk63hxr2B2zun7gdfJxZkmgRl3rNQvCKEGcU0ZgSWBMGq6L9HEnv0/dC5vMebVj7WdEu+P5SyYH0ANroZp1hze042sOmWSHn8DHNK7TGpve9veZ2NYJWp6H1YfrFaOEYv21s8wqtn+wL33/Z67z8LtoUmtIQNo+6aMHIb7to1SGP3bS3JNsq/VhCG7KFn41Oj3GuTAbVzOVKpVxnrqg11MoIyKfvEjzvtulrHG/yvaWf4LWnmu5ifDinwXZw/1Zxn6K+G4OSU+N6OJ62eg95NeW6lpq3h/3fsq98PClGifRoxke9KI5RONz20vVewhL/NCJUediF1olE4hgutuzr68POi07kq9w+DR7fvy1JdHdyVfvoR4x/BdSt+cSRNoWIQUkUd/IN5XXAQTbfFScrZgF9bqPNuUstJTxzJKGFKRBYaU6erLym8zWfuRz0x3efbqMt+XDVSI4cmdL1yAf30IbwFbTlacm15NCuXYBjjmreMAhWcdCu3X5qKkNTm5/uLVybzpXWj/UsGe3C5WxfsnZCu5w4dF23CNYfd3MG122pu7sOWKIQ20hfZwWNomM+xSuPXO+nJcLFx+yxaLpyr9mN8VoCHYKLWna3mctpoanDp43893xDPkkUMteeKbSc5bLX79+nXOsSW3OqsM0yTD//Pd9g14J5bSuiwq2DnBt9hzT9MfI916A4+e/Fhq0uBfvr1mvxuRmrslrgnB7qO0LW/u55g3d/8texc6+xM6y5Rt11512SY6GFKppaRbyD9u7CZR77sz+C9lysbqO9e/Z8PTTOCy/NaF2qtjD1rkJ/LlY31y2o2zTEPNJVDuzaxue6VcatfJ45tey8VfPqcM5vY0ASffqyBj+sr0QBsxTdK+K2Nm59Y2M38WmqwNY+D7sqEm3gYk3ayRi5pf3lgCeIE6d3C+i8L0BRLDQQwShoVkPz6evQzkiXDdj8DSuVMlUM4kAaekOq7nerIgrgnCIY5DjH2Q2Tlk3Vu29Y0akGEj/5vMHUR8E+IXI/9+CdR+7tDR7PzsoUA4CzMqHVGbZmoGBM7iDaDnkFHpXDiWvpqDvgnuB9tIxvJA2iDe0wjBmmndHSdjbImFffQquZRD34HZzeyYFZcCxZHf2HWJDU8BgH7Ndtj025h3HU6HkH2azitveF8/HYo49HtzVryt2W6JJBLfRBdM7daQt8teU0IuFrn4cLPczVDa0K70hdZ/ozJEyPPPLII4888sgjjzzyyCOPPMKUfCBD8O5WeN9LpMJCvJbvCX+xAKbep2Qc/ki8qFZSdlO1MNc7iQVtAQP4gJQbtOuv6Yk3XPzKntHCyC9Ay5/bjPaFeOjHXza3IOCthTblmJGQ6+ULq5zidYEWvyoRp1aYD1qAV1wX8cl4Q/wUrzWdoEttxS7oWyo0x/T9EZEOi8bXKv5UP4NWRSWjKlK8UvkSFcqXEel7RVsqsIQWpIR3vE3u0cI7vixo4893fOCdgBDaV8Qau3vH1RKHFH/zDf7TsQuTuvkBSya0cb2+1rlNlzSV6nJuhXqLeIF/t2hxY/qV0b5wJD9HChAtUGtCKvGmniEPHPMA2zeipZlMaMtlHS3OBBC2HtrNb19roP1JL5UsGbLJCble7ix5ixafQpWNME200QCmdyqOb9FuLVli7/gVsR+15FfZjcGVlS+rXoomReZFnu++QnvwUgXtb/FSf4b8bh7+MXmRy9b/CFpctP/M1GKcKvX0yCOPPPLI/0P+E2AAA7JN+/OU8bgAAAAASUVORK5CYII="

return image1;
}