如何透過 HTML5 Canvas 高品質(zhì)縮小影像？

傳統(tǒng)方法使用內(nèi)插法會導致影像品質(zhì)下降。要達到最佳質(zhì)量，請執(zhí)行以下步驟：

1. 停用插值：

canvas, img {
    image-rendering: optimizeQuality;
    image-rendering: -moz-crisp-edges;
    image-rendering: -webkit-optimize-contrast;
    image-rendering: optimize-contrast;
    -ms-interpolation-mode: nearest-neighbor;
}

2. 逐步縮小影像：

• 分成多個步驟縮小影像，而不是一次性大幅縮小。
• 例如，將影像縮小到 50% 大小，再逐步縮小到目標尺寸。

3. 使用像素級演算法（非插值演算法）：

• 插值演算法會模糊影像，而像素級演算法會考慮每個來源像素對目標像素的貢獻。
• 考慮使用 GameAlchemist 開發(fā)的演算法，該演算法在逐步縮小時提供更佳的品質(zhì)。

程式碼樣本：

// GameAlchemist 算法
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
    /* untested !
    var sA = 0;  //source alpha  */    

    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];

            /* !! untested : handling alpha !!
               sA = sBuffer[sIndex + 3];
               if (!sA) continue;
               if (sA != 0xFF) {
                   sR = (sR * sA) >> 8;  // or use /256 instead ??
                   sG = (sG * sA) >> 8;
                   sB = (sB * sA) >> 8;
               }
            */
            if (!crossX &amp;&amp; !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 &amp;&amp; !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 &amp;&amp; !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; //  

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