Kanvas HTML5: Menurunkan Penskalaan Imej dengan Kualiti Tinggi

Masalah: Menurunkan skala imej menggunakan kanvas HTML5 menghasilkan kualiti yang rendah output walaupun mendayakan pengoptimuman pemaparan imej.

Penyelesaian:

Menurunkan skala imej dalam kanvas HTML5 melibatkan penukaran berbilang piksel dalam imej asal kepada satu piksel dalam imej berskala. Teknik penurunan skala biasa dalam penyemak imbas mengendalikan proses ini dengan tidak cekap, yang membawa kepada kehilangan butiran dan bunyi.

Untuk mencapai penskalaan rendah berkualiti tinggi, pertimbangkan untuk menggunakan algoritma sempurna piksel yang mengira sumbangan semua piksel sumber ke destinasi dengan tepat piksel.

Algoritma:

<code class="javascript">// scales the canvas by (float) scale < 1
// returns a new canvas containing the scaled image.
function downScaleCanvas(cv, scale) {
    if (!(scale < 1) || !(scale > 0)) 
        throw ('scale must be a positive number <1 ');

    var sqScale = scale * scale; // square scale = area of source pixel within target
    var sw = cv.width; // source image width
    var sh = cv.height; // source image height
    var tw = Math.floor(sw * scale); // target image width
    var th = Math.floor(sh * scale); // target image height
    var sx = 0, sy = 0, sIndex = 0; // source x,y, index within source array
    var tx = 0, ty = 0, yIndex = 0, tIndex = 0; // target x,y, x,y index within target array
    var tX = 0, tY = 0; // rounded tx, ty
    var w = 0, nw = 0, wx = 0, nwx = 0, wy = 0, nwy = 0; // weight / next weight x / y
    // weight is weight of current source point within target.
    // next weight is weight of current source point within next target's point.
    var crossX = false; // does scaled px cross its current px right border ?
    var crossY = false; // does scaled px cross its current px bottom border ?
    var sBuffer = cv.getContext('2d').
        getImageData(0, 0, sw, sh).data; // source buffer 8 bit rgba
    var tBuffer = new Float32Array(3 * tw * th); // target buffer Float32 rgb
    var sR = 0, sG = 0,  sB = 0; // source's current point r,g,b

    for (sy = 0; sy < sh; sy++) {
        ty = sy * scale; // y src position within target
        tY = 0 | ty;     // rounded : target pixel's y
        yIndex = 3 * tY * tw;  // line index within target array
        crossY = (tY != (0 | ty + scale)); 
        if (crossY) { // if pixel is crossing botton target pixel
            wy = (tY + 1 - ty); // weight of point within target pixel
            nwy = (ty + scale - tY - 1); // ... within y+1 target pixel
        }
        for (sx = 0; sx < sw; sx++, sIndex += 4) {
            tx = sx * scale; // x src position within target
            tX = 0 | ?tx;    // rounded : target pixel's x
            tIndex = yIndex + tX * 3; // target pixel index within target array
            crossX = (tX != (0 | tx + scale));
            if (crossX) { // if pixel is crossing target pixel's right
                wx = (tX + 1 - tx); // weight of point within target pixel
                nwx = (tx + scale - tX - 1); // ... within x+1 target pixel
            }
            sR = sBuffer[sIndex    ];   // retrieving r,g,b for curr src px.
            sG = sBuffer[sIndex + 1];
            sB = sBuffer[sIndex + 2];

            if (!crossX && !crossY) { // pixel does not cross
                // just add components weighted by squared scale.
                tBuffer[tIndex    ] += sR * sqScale;
                tBuffer[tIndex + 1] += sG * sqScale;
                tBuffer[tIndex + 2] += sB * sqScale;
            } else if (crossX && !crossY) { // cross on X only
                w = wx * scale;
                // add weighted component for current px
                tBuffer[tIndex    ] += sR * w;
                tBuffer[tIndex + 1] += sG * w;
                tBuffer[tIndex + 2] += sB * w;
                // add weighted component for next (tX+1) px                
                nw = nwx * scale
                tBuffer[tIndex + 3] += sR * nw;
                tBuffer[tIndex + 4] += sG * nw;
                tBuffer[tIndex + 5] += sB * nw;
            } else if (crossY && !crossX) { // cross on Y only
                w = wy * scale;
                // add weighted component for current px
                tBuffer[tIndex    ] += sR * w;
                tBuffer[tIndex + 1] += sG * w;
                tBuffer[tIndex + 2] += sB * w;
                // add weighted component for next (tY+1) px                
                nw = nwy * scale
                tBuffer[tIndex + 3 * tw    ] += sR * nw;
                tBuffer[tIndex + 3 * tw + 1] += sG * nw;
                tBuffer[tIndex + 3 * tw + 2] += sB * nw;
            } else { // crosses both x and y : four target points involved
                // add weighted component for current px
                w = wx * wy;
                tBuffer[tIndex    ] += sR * w;
                tBuffer[tIndex + 1] += sG * w;
                tBuffer[tIndex + 2] += sB * w;
                // for tX + 1; tY px
                nw = nwx * wy;
                tBuffer[tIndex + 3] += sR * nw;
                tBuffer[tIndex + 4] += sG * nw;
                tBuffer[tIndex + 5] += sB * nw;
                // for tX ; tY + 1 px
                nw = wx * nwy;
                tBuffer[tIndex + 3 * tw    ] += sR * nw;
                tBuffer[tIndex + 3 * tw + 1] += sG * nw;
                tBuffer[tIndex + 3 * tw + 2] += sB * nw;
                // for tX + 1 ; tY +1 px
                nw = nwx * nwy;
                tBuffer[tIndex + 3 * tw + 3] += sR * nw;
                tBuffer[tIndex + 3 * tw + 4] += sG * nw;
                tBuffer[tIndex + 3 * tw + 5] += sB * nw;
            }
        } // end for sx 
    } // end for sy

    // create result canvas
    var resCV = document.createElement('canvas');
    resCV.width = tw;
    resCV.height = th;
    var resCtx = resCV.getContext('2d');
    var imgRes = resCtx.getImageData(0, 0, tw, th);
    var tByteBuffer = imgRes.data;
    // convert float32 array into a UInt8Clamped Array
    var pxIndex = 0; //  
    for (sIndex = 0, tIndex = 0; pxIndex < tw * th; sIndex += 3, tIndex += 4, pxIndex++) {
        tByteBuffer[tIndex] = Math.ceil(tBuffer[sIndex]);
        tByteBuffer[tIndex + 1] = Math.ceil(tBuffer[sIndex + 1]);
        tByteBuffer[tIndex + 2] = Math.ceil(tBuffer[sIndex + 2]);
        tByteBuffer[tIndex + 3] = 255;
    }
    // writing result to canvas.
    resCtx.putImageData(imgRes, 0, 0);
    return resCV;
}</code>

Nota Tambahan:

• Peningkatan juga boleh dicapai dengan algoritma yang sama dengan menggunakan skala yang lebih besar daripada 1.
• Algoritma adalah intensif memori dan mungkin tidak sesuai untuk imej yang sangat besar atau aplikasi masa nyata.
• Untuk imej yang lebih kecil atau penggunaan tidak kritikal kes, kaedah yang lebih mudah seperti menggunakan skala 2X lalai penyemak imbas atau menggunakan interpolasi bilinear mungkin memadai.

Atas ialah kandungan terperinci Bagaimanakah saya boleh mencapai penskalaan rendah imej berkualiti tinggi menggunakan kanvas HTML5, walaupun pengoptimuman pemaparan penyemak imbas yang membawa kepada hasil yang buruk?. Untuk maklumat lanjut, sila ikut artikel berkaitan lain di laman web China PHP!