Use JavaScript and canvas to crop images

This article mainly introduces the method of using JavaScript canvas to realize image cropping. Friends who need it can refer to it

canvas is a tag that allows us to use script drawing. It provides a complete series of attributes. and methods. We can use this to achieve graphics drawing, image processing and even simple animation and game production.

The canvas tag has only two attributes: width and height, which are used to set the width and height of the canvas. If not set through tag attributes or scripts, the default is 300*150;

Okay Okay, the introduction to canvas is here first. Let’s take a look at the code for cropping images using javascript combined with canvas:

var selectObj = null;
function ImageCrop(canvasId, imageSource, x, y, width, height) {
    var canvas = $("#" + canvasId);
    if (canvas.length == 0 && imageSource) {
        return;
    }
    function canvasMouseDown(e) {
        StopSelect(e);
        canvas.css("cursor", "default");
    }
    function canvasMouseMove(e) {
        var canvasOffset = canvas.offset();
        var pageX = e.pageX || event.targetTouches[0].pageX;
        var pageY = e.pageY || event.targetTouches[0].pageY;
        iMouseX = Math.floor(pageX - canvasOffset.left);
        iMouseY = Math.floor(pageY - canvasOffset.top);
        canvas.css("cursor", "default");
        if (selectObj.bDragAll) {
            canvas.css("cursor", "move");
            canvas.data("drag", true);
            var cx = iMouseX - selectObj.px;
            cx = cx < 0 ? 0 : cx;
            mx = ctx.canvas.width - selectObj.w;
            cx = cx > mx ? mx : cx;
            selectObj.x = cx;
            var cy = iMouseY - selectObj.py;
            cy = cy < 0 ? 0 : cy;
            my = ctx.canvas.height - selectObj.h;
            cy = cy > my ? my : cy;
            selectObj.y = cy;
        }
        for (var i = 0; i < 4; i++) {
            selectObj.bHow[i] = false;
            selectObj.iCSize[i] = selectObj.csize;
        }
        // hovering over resize cubes
        if (iMouseX > selectObj.x - selectObj.csizeh && iMouseX < selectObj.x + selectObj.csizeh &&
            iMouseY > selectObj.y - selectObj.csizeh && iMouseY < selectObj.y + selectObj.csizeh) {
            canvas.css("cursor", "pointer");
            selectObj.bHow[0] = true;
            selectObj.iCSize[0] = selectObj.csizeh;
        }
        if (iMouseX > selectObj.x + selectObj.w - selectObj.csizeh && iMouseX < selectObj.x + selectObj.w + selectObj.csizeh &&
            iMouseY > selectObj.y - selectObj.csizeh && iMouseY < selectObj.y + selectObj.csizeh) {
            canvas.css("cursor", "pointer");
            selectObj.bHow[1] = true;
            selectObj.iCSize[1] = selectObj.csizeh;
        }
        if (iMouseX > selectObj.x + selectObj.w - selectObj.csizeh && iMouseX < selectObj.x + selectObj.w + selectObj.csizeh &&
            iMouseY > selectObj.y + selectObj.h - selectObj.csizeh && iMouseY < selectObj.y + selectObj.h + selectObj.csizeh) {
            canvas.css("cursor", "pointer");
            selectObj.bHow[2] = true;
            selectObj.iCSize[2] = selectObj.csizeh;
        }
        if (iMouseX > selectObj.x - selectObj.csizeh && iMouseX < selectObj.x + selectObj.csizeh &&
            iMouseY > selectObj.y + selectObj.h - selectObj.csizeh && iMouseY < selectObj.y + selectObj.h + selectObj.csizeh) {
            canvas.css("cursor", "pointer");
            selectObj.bHow[3] = true;
            selectObj.iCSize[3] = selectObj.csizeh;
        }
        if (iMouseX > selectObj.x && iMouseX < selectObj.x + selectObj.w && iMouseY > selectObj.y && iMouseY < selectObj.y + selectObj.h) {
            canvas.css("cursor", "move");
        }
        // in case of dragging of resize cubes
        var iFW, iFH, iFX, iFY, mx, my;
        if (selectObj.bDrag[0]) {
            iFX = iMouseX - selectObj.px;
            iFY = iMouseY - selectObj.py;
            iFW = selectObj.w + selectObj.x - iFX;
            iFH = selectObj.h + selectObj.y - iFY;
            canvas.data("drag", true);
        }
        if (selectObj.bDrag[1]) {
            iFX = selectObj.x;
            iFY = iMouseY - selectObj.py;
            iFW = iMouseX - selectObj.px - iFX;
            iFH = selectObj.h + selectObj.y - iFY;
            canvas.data("drag", true);
        }
        if (selectObj.bDrag[2]) {
            iFX = selectObj.x;
            iFY = selectObj.y;
            iFW = iMouseX - selectObj.px - iFX;
            iFH = iMouseY - selectObj.py - iFY;
            canvas.data("drag", true);
        }
        if (selectObj.bDrag[3]) {
            iFX = iMouseX - selectObj.px;
            iFY = selectObj.y;
            iFW = selectObj.w + selectObj.x - iFX;
            iFH = iMouseY - selectObj.py - iFY;
            canvas.data("drag", true);
        }
        if (iFW > selectObj.csizeh * 2 && iFH > selectObj.csizeh * 2) {
            selectObj.w = iFW;
            selectObj.h = iFH;
            selectObj.x = iFX;
            selectObj.y = iFY;
        }
        drawScene();
    }
    function canvasMouseOut() {
        $(canvas).trigger("mouseup");
    }
    function canvasMouseUp() {
        selectObj.bDragAll = false;
        for (var i = 0; i < 4; i++) {
            selectObj.bDrag[i] = false;
        }
        canvas.css("cursor", "default");
        canvas.data("select", {
            x: selectObj.x,
            y: selectObj.y,
            w: selectObj.w,
            h: selectObj.h
        });
        selectObj.px = 0;
        selectObj.py = 0;
    }
    function Selection(x, y, w, h) {
        this.x = x; // initial positions
        this.y = y;
        this.w = w; // and size
        this.h = h;
        this.px = x; // extra variables to dragging calculations
        this.py = y;
        this.csize = 4; // resize cubes size
        this.csizeh = 6; // resize cubes size (on hover)
        this.bHow = [false, false, false, false]; // hover statuses
        this.iCSize = [this.csize, this.csize, this.csize, this.csize]; // resize cubes sizes
        this.bDrag = [false, false, false, false]; // drag statuses
        this.bDragAll = false; // drag whole selection
    }
    Selection.prototype.draw = function () {
        ctx.strokeStyle = &#39;#666&#39;;
        ctx.lineWidth = 2;
        ctx.strokeRect(this.x, this.y, this.w, this.h);
        // draw part of original image
        if (this.w > 0 && this.h > 0) {
            ctx.drawImage(image, this.x, this.y, this.w, this.h, this.x, this.y, this.w, this.h);
        }
        // draw resize cubes
        ctx.fillStyle = &#39;#999&#39;;
        ctx.fillRect(this.x - this.iCSize[0], this.y - this.iCSize[0], this.iCSize[0] * 2, this.iCSize[0] * 2);
        ctx.fillRect(this.x + this.w - this.iCSize[1], this.y - this.iCSize[1], this.iCSize[1] * 2, this.iCSize[1] * 2);
        ctx.fillRect(this.x + this.w - this.iCSize[2], this.y + this.h - this.iCSize[2], this.iCSize[2] * 2, this.iCSize[2] * 2);
        ctx.fillRect(this.x - this.iCSize[3], this.y + this.h - this.iCSize[3], this.iCSize[3] * 2, this.iCSize[3] * 2);
    };
    var drawScene = function () {
        ctx.clearRect(0, 0, ctx.canvas.width, ctx.canvas.height); // clear canvas
        // draw source image
        ctx.drawImage(image, 0, 0, ctx.canvas.width, ctx.canvas.height);
        // and make it darker
        ctx.fillStyle = &#39;rgba(0, 0, 0, 0.5)&#39;;
        ctx.fillRect(0, 0, ctx.canvas.width, ctx.canvas.height);
        // draw selection
        selectObj.draw();
        canvas.mousedown(canvasMouseDown);
        canvas.on("touchstart", canvasMouseDown);
    };
    var createSelection = function (x, y, width, height) {
        var content = $("#imagePreview");
        x = x || Math.ceil((content.width() - width) / 2);
        y = y || Math.ceil((content.height() - height) / 2);
        return new Selection(x, y, width, height);
    };
    var ctx = canvas[0].getContext("2d");
    var iMouseX = 1;
    var iMouseY = 1;
    var image = new Image();
    image.onload = function () {
        selectObj = createSelection(x, y, width, height);
        canvas.data("select", {
            x: selectObj.x,
            y: selectObj.y,
            w: selectObj.w,
            h: selectObj.h
        });
        drawScene();
    };
    image.src = imageSource;
    canvas.mousemove(canvasMouseMove);
    canvas.on("touchmove", canvasMouseMove);
    var StopSelect = function (e) {
        var canvasOffset = $(canvas).offset();
        var pageX = e.pageX || event.targetTouches[0].pageX;
        var pageY = e.pageY || event.targetTouches[0].pageY;
        iMouseX = Math.floor(pageX - canvasOffset.left);
        iMouseY = Math.floor(pageY - canvasOffset.top);
        selectObj.px = iMouseX - selectObj.x;
        selectObj.py = iMouseY - selectObj.y;
        if (selectObj.bHow[0]) {
            selectObj.px = iMouseX - selectObj.x;
            selectObj.py = iMouseY - selectObj.y;
        }
        if (selectObj.bHow[1]) {
            selectObj.px = iMouseX - selectObj.x - selectObj.w;
            selectObj.py = iMouseY - selectObj.y;
        }
        if (selectObj.bHow[2]) {
            selectObj.px = iMouseX - selectObj.x - selectObj.w;
            selectObj.py = iMouseY - selectObj.y - selectObj.h;
        }
        if (selectObj.bHow[3]) {
            selectObj.px = iMouseX - selectObj.x;
            selectObj.py = iMouseY - selectObj.y - selectObj.h;
        }
        if (iMouseX > selectObj.x + selectObj.csizeh &&
            iMouseX < selectObj.x + selectObj.w - selectObj.csizeh &&
            iMouseY > selectObj.y + selectObj.csizeh &&
            iMouseY < selectObj.y + selectObj.h - selectObj.csizeh) {
            selectObj.bDragAll = true;
        }
        for (var i = 0; i < 4; i++) {
            if (selectObj.bHow[i]) {
                selectObj.bDrag[i] = true;
            }
        }
    };
    canvas.mouseout(canvasMouseOut);
    canvas.mouseup(canvasMouseUp);
    canvas.on("touchend", canvasMouseUp);
    this.getImageData = function (previewID) {
        var tmpCanvas = $("<canvas></canvas>")[0];
        var tmpCtx = tmpCanvas.getContext("2d");
        if (tmpCanvas && selectObj) {
            tmpCanvas.width = selectObj.w;
            tmpCanvas.height = selectObj.h;
            tmpCtx.drawImage(image, selectObj.x, selectObj.y, selectObj.w, selectObj.h, 0, 0, selectObj.w, selectObj.h);
            if (document.getElementById(previewID)) {
                document.getElementById(previewID).src = tmpCanvas.toDataURL();
                document.getElementById(previewID).style.border = "1px solid #ccc";
            }
            return tmpCanvas.toDataURL();
        }
    };
}
function autoResizeImage(maxWidth, maxHeight, objImg) {
    var img = new Image();
    img.src = objImg.src;
    var hRatio;
    var wRatio;
    var ratio = 1;
    var w = objImg.width;
    var h = objImg.height;
    wRatio = maxWidth / w;
    hRatio = maxHeight / h;
    if (w < maxWidth && h < maxHeight) {
        return;
    }
    if (maxWidth == 0 && maxHeight == 0) {
        ratio = 1;
    } else if (maxWidth == 0) {
        if (hRatio < 1) {
            ratio = hRatio;
        }
    } else if (maxHeight == 0) {
        if (wRatio < 1) {
            ratio = wRatio;
        }
    } else if (wRatio < 1 || hRatio < 1) {
        ratio = (wRatio <= hRatio ? wRatio : hRatio);
    } else {
        ratio = (wRatio <= hRatio ? wRatio : hRatio) - Math.floor(wRatio <= hRatio ? wRatio : hRatio);
    }
    if (ratio < 1) {
        if (ratio < 0.5 && w < maxWidth && h < maxHeight) {
            ratio = 1 - ratio;
        }
        w = w * ratio;
        h = h * ratio;
    }
    objImg.height = h;
    objImg.width = w;
}

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