我比较了在画布图像上进行缩减的两个函数的性能。事实证明，Java函数比JSNI函数慢得多。这很奇怪，因为我假设GWT编译器优化了它至少与JSNI代码一样快的代码。
这是Java函数：
public static final CanvasElement scaleCanvas(CanvasElement cv, double scale) {
if (!(scale < 1) || !(scale > 0)) {
GWT.log("scale must be a positive number <1");
// throw new Exception("scale must be a positive number <1 ");
}
GWT.log("scaleCanvas start");
double sqScale = scale * scale; // square scale = area of source pixel within target
int sw = cv.getWidth(); // source image width
int sh = cv.getHeight(); // source image height
double tw = Math.ceil(sw * scale); // target image width
double th = Math.ceil(sh * scale); // target image height
int sx = 0, sy = 0, sIndex = 0; // source x,y, index within source array
double tx = 0, ty = 0;
int yIndex = 0, tIndex = 0; // target x,y, x,y index within target array
double tX = 0, tY = 0; // rounded tx, ty
double 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.
boolean crossX = false; // does scaled px cross its current px right border ?
boolean crossY = false; // does scaled px cross its current px bottom border ?
CanvasPixelArray sBuffer = cv.getContext2d().getImageData(0, 0, sw, sh).getData(); // source buffer 8 bit rgba
Float32Array tBuffer = TypedArrays.createFloat32Array(4 * sw * sh);
double sR = 0, sG = 0, sB = 0; // source's current point r,g,b
// untested !
double sA = 0; //source alpha
for (sy = 0; sy < sh; sy++) {
GWT.log("sy: "+sy+" sh: "+sh);
ty = sy * scale; // y src position within target
tY = (long)ty; // rounded : target pixel's y // ?????
yIndex = (int)Math.floor(4 * tY * tw); // line index within target array
crossY = (tY != ( (long)(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 = (long)tx; // rounded : target pixel's x // ?????
tIndex = (int)Math.floor(yIndex + tX * 4); // target pixel index within target array // ?????
crossX = (tX != ((int)Math.floor(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.get(sIndex); // retrieving r,g,b for curr src px.
sG = sBuffer.get(sIndex + 1);
sB = sBuffer.get(sIndex + 2);
sA = sBuffer.get(sIndex + 3);
if (!crossX && !crossY) { // pixel does not cross
// just add components weighted by squared scale.
tBuffer.set(tIndex , (float)(tBuffer.get(tIndex) + sR * sqScale));
tBuffer.set(tIndex + 1, (float)(tBuffer.get(tIndex + 1) + sG * sqScale));
tBuffer.set(tIndex + 2, (float)(tBuffer.get(tIndex + 2) + sB * sqScale));
tBuffer.set(tIndex + 3, (float)(tBuffer.get(tIndex + 3) + sA * sqScale));
} else if (crossX && !crossY) { // cross on X only
w = wx * scale;
// add weighted component for current px
tBuffer.set(tIndex , (float)(tBuffer.get(tIndex) + sR * w));
tBuffer.set(tIndex + 1, (float)(tBuffer.get(tIndex + 1) + sG * w));
tBuffer.set(tIndex + 2, (float)(tBuffer.get(tIndex + 2) + sB * w));
tBuffer.set(tIndex + 3, (float)(tBuffer.get(tIndex + 3) + sA * w));
// add weighted component for next (tX+1) px
nw = nwx * scale;
tBuffer.set(tIndex + 4, (float)(tBuffer.get(tIndex + 4) + sR * nw)); // not 3
tBuffer.set(tIndex + 5, (float)(tBuffer.get(tIndex + 5) + sG * nw)); // not 4
tBuffer.set(tIndex + 6, (float)(tBuffer.get(tIndex + 6) + sB * nw)); // not 5
tBuffer.set(tIndex + 7, (float)(tBuffer.get(tIndex + 7) + sA * nw)); // not 6
} else if (crossY && !crossX) { // cross on Y only
w = wy * scale;
// add weighted component for current px
tBuffer.set(tIndex , (float)(tBuffer.get(tIndex) + sR * w));
tBuffer.set(tIndex + 1, (float)(tBuffer.get(tIndex + 1) + sG * w));
tBuffer.set(tIndex + 2, (float)(tBuffer.get(tIndex + 2) + sB * w));
tBuffer.set(tIndex + 3, (float)(tBuffer.get(tIndex + 3) + sA * w));
// add weighted component for next (tY+1) px
nw = nwy * scale;
tBuffer.set((int)Math.floor(tIndex + 4 * tw) , (float)(tBuffer.get((int)Math.floor(tIndex + 4 * tw)) + sR * nw)); // *4, not 3
tBuffer.set((int)Math.floor(tIndex + 4 * tw + 1), (float)(tBuffer.get((int)Math.floor(tIndex + 4 * tw + 1)) + sG * nw)); // *4, not 3
tBuffer.set((int)Math.floor(tIndex + 4 * tw + 2), (float)(tBuffer.get((int)Math.floor(tIndex + 4 * tw + 2)) + sB * nw)); // *4, not 3
tBuffer.set((int)Math.floor(tIndex + 4 * tw + 3), (float)(tBuffer.get((int)Math.floor(tIndex + 4 * tw + 3)) + sA * nw)); // *4, not 3
} else { // crosses both x and y : four target points involved
// add weighted component for current px
w = wx * wy;
tBuffer.set(tIndex , (float)(tBuffer.get(tIndex) + sR * w));
tBuffer.set(tIndex + 1, (float)(tBuffer.get(tIndex + 1) + sG * w));
tBuffer.set(tIndex + 2, (float)(tBuffer.get(tIndex + 2) + sB * w));
tBuffer.set(tIndex + 3, (float)(tBuffer.get(tIndex + 3) + sA * w));
// for tX + 1; tY px
nw = nwx * wy;
tBuffer.set(tIndex + 4, (float)(tBuffer.get(tIndex + 4) + sR * nw)); // same for x
tBuffer.set(tIndex + 5, (float)(tBuffer.get(tIndex + 5) + sG * nw));
tBuffer.set(tIndex + 6, (float)(tBuffer.get(tIndex + 6) + sB * nw));
tBuffer.set(tIndex + 7, (float)(tBuffer.get(tIndex + 7) + sA * nw));
// for tX ; tY + 1 px
nw = wx * nwy;
tBuffer.set((int)Math.floor(tIndex + 4 * tw) , (float)(tBuffer.get((int)Math.floor(tIndex + 4 * tw)) + sR * nw)); // same for mul
tBuffer.set((int)Math.floor(tIndex + 4 * tw + 1), (float)(tBuffer.get((int)Math.floor(tIndex + 4 * tw + 1)) + sG * nw));
tBuffer.set((int)Math.floor(tIndex + 4 * tw + 2), (float)(tBuffer.get((int)Math.floor(tIndex + 4 * tw + 2)) + sB * nw));
tBuffer.set((int)Math.floor(tIndex + 4 * tw + 3), (float)(tBuffer.get((int)Math.floor(tIndex + 4 * tw + 3)) + sA * nw));
// for tX + 1 ; tY +1 px
nw = nwx * nwy;
tBuffer.set((int)Math.floor(tIndex + 4 * tw + 4), (float)(tBuffer.get((int)Math.floor(tIndex + 4 * tw + 4)) + sR * nw)); // same for both x and y
tBuffer.set((int)Math.floor(tIndex + 4 * tw + 5), (float)(tBuffer.get((int)Math.floor(tIndex + 4 * tw + 5)) + sG * nw));
tBuffer.set((int)Math.floor(tIndex + 4 * tw + 6), (float)(tBuffer.get((int)Math.floor(tIndex + 4 * tw + 6)) + sB * nw));
tBuffer.set((int)Math.floor(tIndex + 4 * tw + 7), (float)(tBuffer.get((int)Math.floor(tIndex + 4 * tw + 7)) + sA * nw));
}
} // end for sx
} // end for sy
// create result canvas
Canvas resCV = Canvas.createIfSupported();
resCV.getCanvasElement().setWidth((int)Math.floor(tw));
resCV.getCanvasElement().setHeight((int)Math.floor(th));
Context2d resCtx = resCV.getContext2d();
ImageData imgRes = resCtx.getImageData(0, 0, tw, th);
CanvasPixelArray tByteBuffer = imgRes.getData();
// convert float32 array into a UInt8Clamped Array
int pxIndex = 0;
for (sIndex = 0, tIndex = 0; pxIndex < tw * th; sIndex += 4, tIndex += 4, pxIndex++) {
tByteBuffer.set(tIndex, (int)Math.ceil(tBuffer.get(sIndex)));
tByteBuffer.set(tIndex + 1, (int)Math.ceil(tBuffer.get(sIndex + 1)));
tByteBuffer.set(tIndex + 2, (int)Math.ceil(tBuffer.get(sIndex + 2)));
tByteBuffer.set(tIndex + 3, (int)Math.ceil(tBuffer.get(sIndex + 3)));
}
// writing result to canvas.
resCtx.putImageData(imgRes, 0, 0);
return resCV.getCanvasElement();
}
这是等效的JSNI函数：
public static final native CanvasElement resizeCanvas(CanvasElement cv, double 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.ceil(sw * scale); // target image width
var th = Math.ceil(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(4 * sw * sh); // 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 = 4 * 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 * 4; // 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];
sA = sBuffer[sIndex + 3];
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;
tBuffer[tIndex + 3] += sA * 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;
tBuffer[tIndex + 3] += sA * w;
// add weighted component for next (tX+1) px
nw = nwx * scale;
tBuffer[tIndex + 4] += sR * nw; // not 3
tBuffer[tIndex + 5] += sG * nw; // not 4
tBuffer[tIndex + 6] += sB * nw; // not 5
tBuffer[tIndex + 7] += sA * nw; // not 6
} 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;
tBuffer[tIndex + 3] += sA * w;
// add weighted component for next (tY+1) px
nw = nwy * scale;
tBuffer[tIndex + 4 * tw ] += sR * nw; // *4, not 3
tBuffer[tIndex + 4 * tw + 1] += sG * nw; // *4, not 3
tBuffer[tIndex + 4 * tw + 2] += sB * nw; // *4, not 3
tBuffer[tIndex + 4 * tw + 3] += sA * nw; // *4, not 3
} 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;
tBuffer[tIndex + 3] += sA * w;
// for tX + 1; tY px
nw = nwx * wy;
tBuffer[tIndex + 4] += sR * nw; // same for x
tBuffer[tIndex + 5] += sG * nw;
tBuffer[tIndex + 6] += sB * nw;
tBuffer[tIndex + 7] += sA * nw;
// for tX ; tY + 1 px
nw = wx * nwy;
tBuffer[tIndex + 4 * tw ] += sR * nw; // same for mul
tBuffer[tIndex + 4 * tw + 1] += sG * nw;
tBuffer[tIndex + 4 * tw + 2] += sB * nw;
tBuffer[tIndex + 4 * tw + 3] += sA * nw;
// for tX + 1 ; tY +1 px
nw = nwx * nwy;
tBuffer[tIndex + 4 * tw + 4] += sR * nw; // same for both x and y
tBuffer[tIndex + 4 * tw + 5] += sG * nw;
tBuffer[tIndex + 4 * tw + 6] += sB * nw;
tBuffer[tIndex + 4 * tw + 7] += sA * 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 += 4, 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] = Math.ceil(tBuffer[sIndex + 3]);
}
// writing result to canvas.
resCtx.putImageData(imgRes, 0, 0);
return resCV;
}-*/;
为什么Java函数比JSNI函数慢得多？
答案 0 :(得分：0)
你是否已经编译并在＆＃34; java场景中运行了应用程序的编译版本＆＃34; ？ 因为，如果你没有，只是从Eclipse Run-＆gt; Web应用程序运行。 GWT将在运行时转换javascript中的java代码，这比编译版本慢很多。