Rotate image and fit show use canvas

Description

In the field of image processing, We always to show image after modified the image raw data.It is very easy with using c, c++ and other compiled programming language. But, if you use the interpreted programming language like javascript. i think it will maybe more complicated.But, We always can find work way.

The below code is not all, Just some code snippet.

Code snippet

Here rotation ways use the temporary variable to storage temporary image every rotated. before rotatetion, should use src image.

var RotateImg = {
    srcImg:"",          // storage src img path, when you begin to rotate every time, you should use src image that can reduce the error of each roataiton
    tmpImg:"",          // storage temp image oath after rotated src image.
};

/** Use the canvas to rotate image raw data to show */
RotateImg.rotate = function(nAngle)
{
    var Img = new Image();
    Img.addEventListener("load",function(){
        var imgW = Img.width;
        var imgH = Img.height;

        var cvs = document.createElement("canvas");
        var context = cvs.getContext( "2d" );
        var degree = nAngle * Math.PI / 180;
        switch ( nAngle )
        {
            case 0://Clockwise 0
                cvs.width  = imgW;
                cvs.height = imgH;
                context.drawImage( Img, 0, 0, imgW, imgH );
                break;
            case 90://Clockwise 90
                cvs.width  = imgH;
                cvs.height = imgW; 
                context.rotate( degree );
                context.drawImage( Img, 0, -imgH, imgW, imgH );
                break;
            case 180://Clockwise 180
                cvs.width  = imgW;
                cvs.height = imgH;
                context.rotate( degree );
                context.drawImage( Img, -imgW, -imgH, imgW, imgH );
                break;
            case 270://Clockwise 270
                cvs.width  = imgH;
                cvs.height = imgW;
                context.rotate( degree );
                context.drawImage( Img, -imgW, 0, imgW, imgH );
                break;
        }

        /** jpeg level [0,1] */
        var dataBase64 = cvs.toDataURL( "image/jpeg", 0.8 );

        /** remove base64 description information before image data.---->data:image/png;base64,iVBORw0KG... */
        dataBase64 = dataBase64.split( "," )[1];
        dataBase64 = window.atob( dataBase64 );
        var ia = new Uint8Array( dataBase64.length );
        for ( var i = 0; i < dataBase64.length; i++ ) {
            ia[i] = dataBase64.charCodeAt( i );
        };

        var blob = new Blob( [ia], {type:"image/jpeg"} );

        /** storage temp image path in member */
        RotateImg.tmpImg = window.URL.createObjectURL( blob );
        RotateImg.showImg(RotateImg.tmpImg);
    },false);

    Img.src = RotateImg.srcImg;
};

Here another rotation ways not use the temporary variable, Just use the src. This way is more complex.

RotateImg.rotateExampleImage = function( nAngle )
{
    imgLoaded = true;
    var  img = new Image();

    img.onload = function()
    {
        var imgW = img.width;
        var imgH = img.height;
        var degree = nAngle * Math.PI / 180;
        var cvs  = document.getElementById( "canvas" );
        var context = cvs.getContext( "2d" );

        switch ( nAngle ) 
        {
            case 0://Clockwise 0
                var xyImgScale    = imgW / imgH;
                var xyCanvasScale = cvs.width / cvs.height;
                var finalHeight   = 0;
                var finalWidth    = 0;
                if( xyImgScale >= xyCanvasScale ) {
                    finalWidth = cvs.width * 1;
                    finalHeight = finalWidth / xyImgScale;
                } else {
                    finalHeight = cvs.height * 1;
                    finalWidth = finalHeight * xyImgScale;
                }
                var xPos = ( cvs.width - finalWidth   ) / 2;
                var yPos = ( cvs.height - finalHeight ) / 2;

                var maxV = cvs.width > cvs.height ? cvs.width : cvs.height;
                context.clearRect( -maxV, -maxV, 2*maxV, 2*maxV );
                context.rotate( degree );
                context.drawImage( img, 0, 0, imgW, imgH, xPos, yPos,finalWidth, finalHeight );
                context.rotate( -degree );
                break;
            case 90://Clockwise 90
                var xyImgScale    = imgW / imgH;
                var xyCanvasScale = cvs.height / cvs.width;
                var finalHeight   = 0;
                var finalWidth    = 0;
                if( xyImgScale >= xyCanvasScale ) {
                    finalWidth = cvs.height * 1;
                    finalHeight = finalWidth / xyImgScale;
                } else {
                    finalHeight = cvs.width * 1;
                    finalWidth = finalHeight * xyImgScale;
                }
                var xPos = ( cvs.height - finalWidth   ) / 2;
                var yPos = ( cvs.width - finalHeight ) / 2;

                var maxV = cvs.width > cvs.height ? cvs.width : cvs.height;
                context.clearRect( -maxV, -maxV, 2*maxV, 2*maxV );
                context.rotate( degree );
                context.drawImage( img, 0, 0, imgW, imgH, xPos, yPos-cvs.width , finalWidth,finalHeight );
                yPos = yPos-cvs.width;
                context.rotate( -degree );
                break;
            case 180://Clockwise 180
                var xyImgScale    = imgW / imgH;
                var xyCanvasScale = cvs.width / cvs.height;
                var finalHeight   = 0;
                var finalWidth    = 0;
                if( xyImgScale >= xyCanvasScale ) {
                    finalWidth = cvs.width * 1;
                    finalHeight = finalWidth / xyImgScale;
                } else {
                    finalHeight = cvs.height * 1;
                    finalWidth = finalHeight * xyImgScale;
                }
                var xPos = ( cvs.width - finalWidth   ) / 2;
                var yPos = ( cvs.height - finalHeight ) / 2;

                var maxV = cvs.width > cvs.height ? cvs.width : cvs.height;
                context.clearRect( -maxV, -maxV, 2*maxV, 2*maxV );

                context.rotate( degree );
                context.drawImage( img, 0, 0, imgW, imgH, xPos-cvs.width, yPos-cvs.height,finalWidth, finalHeight );
                context.rotate( -degree );
                break;
            case 270://Clockwise 270
                var xyImgScale    = imgW / imgH;
                var xyCanvasScale = cvs.height / cvs.width;
                var finalHeight   = 0;
                var finalWidth    = 0;
                if( xyImgScale >= xyCanvasScale ) {
                    finalWidth = cvs.height * 1;
                    finalHeight = finalWidth / xyImgScale;
                } else {
                    finalHeight = cvs.width * 1;
                    finalWidth = finalHeight * xyImgScale;
                }
                var xPos = ( cvs.height - finalWidth   ) / 2;
                var yPos = ( cvs.width - finalHeight ) / 2;

                var maxV = cvs.width > cvs.height ? cvs.width : cvs.height;
                context.clearRect( -maxV, -maxV, 2*maxV, 2*maxV );

                context.rotate( degree );
                context.drawImage( img, 0, 0, imgW, imgH, xPos-cvs.height, yPos, finalWidth,finalHeight );
                context.rotate( -degree );
                break;
        }

    };

    img.src = RotateImg.srcImg;
}

Fit show image in view rect center

RotateImg.showImg = function( imgSrc )
{
    var img = new Image();
    var loaded = false;

    function loadHandler() {
        if (loaded) {
            return;
        }

        loaded = true;

        /** core code for fit view rect to show image */
        var imgW = img.width;
        var imgH = img.height;
        var xyImgScale  = imgW / imgH;

        var mycvs  = document.getElementById( "canvas" );
        var xyCanvasScale = mycvs.width / mycvs.height;

        var finalHeight = 0;
        var finalWidth  = 0;

        if( xyImgScale >= xyCanvasScale ){
            finalWidth  = mycvs.width * 1;
            finalHeight = finalWidth / xyImgScale;
        }else{
            finalHeight = mycvs.height * 1;
            finalWidth  = finalHeight * xyImgScale;
        }

        var xPos = ( mycvs.width - finalWidth  ) / 2; 
        var yPos = ( mycvs.height - finalHeight ) / 2;

        var context = mycvs.getContext( "2d" );
        context.clearRect( 0, 0, mycvs.width, mycvs.height );
        context.drawImage( img, 0, 0, img.width, img.height, xPos, yPos,finalWidth, finalHeight );
    }

    img.addEventListener('load', loadHandler);
    img.addEventListener('error', function(event){
        //alert("error:");
    });

    img.src = imgSrc;

    if (img.complete) {
        loadHandler();
    }

};

Reference

segmentfault
stackoverflow
stackoverflow
stackoverflow

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