You can not always redraw the canvas, you may have used filters that can not be reversed, or just use so many fill and stroke calls it would be impractical to redraw.
I have my own flood fill based on a simple fill stack that paints to a tolerances and does its best to lessen anti-aliasing artifacts. Unfortunately if you have anti-aliasing on repeated fills will grow the filled region.
Below is the function, adapt it as suited, it is a direct lift from my code with comments added.
// posX,posY are the fill start position. The pixel at the location is used to test tolerance.
// RGBA is the fill colour as an array of 4 bytes all ranged 0-255 for R,G,B,A
// diagonal if true the also fill into pixels that touch at the corners.
// imgData canvas pixel data from ctx.getImageData method
// tolerance Fill tolerance range 0 only allow exact same colour to fill to 255
// fill all but the extreme opposite.
// antiAlias if true fill edges to reduce anti-Aliasing artifacts.
Bitmaps.prototype.floodFill = function (posX, posY, RGBA, diagonal,imgData,tolerance,antiAlias) {
var data = imgData.data; // image data to fill;
antiAlias = true;
var stack = []; // paint stack to find new pixels to paint
var lookLeft = false; // test directions
var lookRight = false;
var w = imgData.width; // width and height
var h = imgData.height;
var painted = new Uint8ClampedArray(w*h); // byte array to mark painted area;
var dw = w*4; // data width.
var x = posX; // just short version of pos because I am lazy
var y = posY;
var ind = y * dw + x * 4; // get the starting pixel index
var sr = data[ind]; // get the start colour tha we will use tollerance against.
var sg = data[ind+1];
var sb = data[ind+2];
var sa = data[ind+3];
var sp = 0;
var dontPaint = false; // flag to indicate if checkColour can paint
// function checks a pixel colour passes tollerance, is painted, or out of bounds.
// if the pixel is over tollerance and not painted set it do reduce anti alising artifacts
var checkColour = function(x,y){
if( x<0 || y < 0 || y >=h || x >= w){ // test bounds
return false;
}
var ind = y * dw + x * 4; // get index of pixel
var dif = Math.max( // get the max channel differance;
Math.abs(sr-data[ind]),
Math.abs(sg-data[ind+1]),
Math.abs(sb-data[ind+2]),
Math.abs(sa-data[ind+3])
);
if(dif < tolerance){ // if under tollerance pass it
dif = 0;
}
var paint = Math.abs(sp-painted[y * w + x]); // is it already painted
if(antiAlias && !dontPaint){ // mitigate anti aliasing effect
// if failed tollerance and has not been painted set the pixel to
// reduce anti alising artifact
if(dif !== 0 && paint !== 255){
data[ind] = RGBA[0];
data[ind+1] = RGBA[1];
data[ind+2] = RGBA[2];
data[ind+3] = (RGBA[3]+data[ind+3])/2; // blend the alpha channel
painted[y * w + x] = 255; // flag pixel as painted
}
}
return (dif+paint)===0?true:false; // return tollerance status;
}
// set a pixel and flag it as painted;
var setPixel = function(x,y){
var ind = y * dw + x * 4; // get index;
data[ind] = RGBA[0]; // set RGBA
data[ind+1] = RGBA[1];
data[ind+2] = RGBA[2];
data[ind+3] = RGBA[3];
painted[y * w + x] = 255; // 255 or any number >0 will do;
}
stack.push([x,y]); // push the first pixel to paint onto the paint stack
while (stack.length) { // do while pixels on the stack
var pos = stack.pop(); // get the pixel
x = pos[0];
y = pos[1];
dontPaint = true; // turn off anti alising
while (checkColour(x,y-1)) { // find the bottom most pixel within tolerance;
y -= 1;
}
dontPaint = false; // turn on anti alising if being used
//checkTop left and right if alowing diagonal painting
if(diagonal){
if(!checkColour(x-1,y) && checkColour(x-1,y-1)){
stack.push([x-1,y-1]);
}
if(!checkColour(x+1,y) && checkColour(x+1,y-1)){
stack.push([x+1,y-1]);
}
}
lookLeft = false; // set look directions
lookRight = false; // only look is a pixel left or right was blocked
while (checkColour(x,y)) { // move up till no more room
setPixel(x,y); // set the pixel
if (checkColour(x - 1,y)) { // check left is blocked
if (!lookLeft) {
stack.push([x - 1, y]); // push a new area to fill if found
lookLeft = true;
}
} else
if (lookLeft) {
lookLeft = false;
}
if (checkColour(x+1,y)) { // check right is blocked
if (!lookRight) {
stack.push([x + 1, y]); // push a new area to fill if found
lookRight = true;
}
} else
if (lookRight) {
lookRight = false;
}
y += 1; // move up one pixel
}
// check down left
if(diagonal){ // check for diagnal areas and push them to be painted
if(checkColour(x-1,y) && !lookLeft){
stack.push([x-1,y]);
}
if(checkColour(x+1,y) && !lookRight){
stack.push([x+1,y]);
}
}
}
// all done
}
There is a better way that gives high quality results, the above code can be adapted to do this by using the painted array to mark the paint edges and then after the fill has completed scan the painted array and apply a convolution filter to each edge pixel you have marked. The filter is directional (depending on which sides are painted) and the code too long for this answer. I have pointed you in the right direction and the infrastructure is above.
Another way to improve the image quality is to super sample the image you are drawing to. Hold a second canvas that is double the size of the image being painted. Do all you drawing to that image and display it to the user on another canvas with CTX.imageSmoothingEnabled
and ctx.setTransform(0.5,0,0,0.5,0,0)
half size, when done and the image is ready half its size manually with the following code (don't rely on canvas imageSmoothingEnabled as it gets it wrong.)
Doing this will greatly improve the quality of your final image and with the above fill almost completely eliminate anti-aliasing artifacts from flood fills.
// ctxS is the source canvas context
var w = ctxS.canvas.width;
var h = ctxS.canvas.height;
var data = ctxS.getImageData(0,0,w,h);
var d = data.data;
var x,y;
var ww = w*4;
var ww4 = ww+4;
for(y = 0; y < h; y+=2){
for(x = 0; x < w; x+=2){
var id = y*ww+x*4;
var id1 = Math.floor(y/2)*ww+Math.floor(x/2)*4;
d[id1] = Math.sqrt((d[id]*d[id]+d[id+4]*d[id+4]+d[id+ww]*d[id+ww]+d[id+ww4]*d[id+ww4])/4);
id += 1;
id1 += 1;
d[id1] = Math.sqrt((d[id]*d[id]+d[id+4]*d[id+4]+d[id+ww]*d[id+ww]+d[id+ww4]*d[id+ww4])/4);
id += 1;
id1 += 1;
d[id1] = Math.sqrt((d[id]*d[id]+d[id+4]*d[id+4]+d[id+ww]*d[id+ww]+d[id+ww4]*d[id+ww4])/4);
id += 1;
id1 += 1;
d[id1] = Math.sqrt((d[id]*d[id]+d[id+4]*d[id+4]+d[id+ww]*d[id+ww]+d[id+ww4]*d[id+ww4])/4);
}
}
ctxS.putImageData(data,0,0); // save imgData
// grab it again for new image we don't want to add artifacts from the GPU
var data = ctxS.getImageData(0,0,Math.floor(w/2),Math.floor(h/2));
var canvas = document.createElement("canvas");
canvas.width = Math.floor(w/2);
canvas.height =Math.floor(h/2);
var ctxS = canvas.getContext("2d",{ alpha: true });
ctxS.putImageData(data,0,0);
// result canvas with downsampled high quality image.