Improve heatmap rendering performance when zsmooth is false

draftlogs/6574_change.md

@@ -0,0 +1 @@
+ - Improve heatmap rendering performance when `zsmooth` is set to false [[#6574](https://github.com/plotly/plotly.js/pull/6574)], with thanks to @lvlte for the contribution!

src/constants/pixelated_image.js

@@ -0,0 +1,22 @@
+'use strict';
+
+// Pixelated image rendering
+// The actual CSS declaration is prepended with fallbacks for older browsers.
+// NB. IE's `-ms-interpolation-mode` works only with <img> not with SVG <image>
+// https://developer.mozilla.org/en-US/docs/Web/CSS/image-rendering
+// https://caniuse.com/?search=image-rendering
+// http://phrogz.net/tmp/canvas_image_zoom.html
+
+exports.CSS_DECLARATIONS = [
+    ['image-rendering', 'optimizeSpeed'],
+    ['image-rendering', '-moz-crisp-edges'],
+    ['image-rendering', '-o-crisp-edges'],
+    ['image-rendering', '-webkit-optimize-contrast'],
+    ['image-rendering', 'optimize-contrast'],
+    ['image-rendering', 'crisp-edges'],
+    ['image-rendering', 'pixelated']
+];
+
+exports.STYLE = exports.CSS_DECLARATIONS.map(function(d) {
+    return d.join(': ') + '; ';
+}).join('');

src/lib/supports_pixelated_image.js

@@ -0,0 +1,42 @@
+'use strict';
+
+var constants = require('../constants/pixelated_image');
+var Drawing = require('../components/drawing');
+var Lib = require('../lib');
+
+var _supportsPixelated = null;
+
+/**
+ * Check browser support for pixelated image rendering
+ *
+ * @return {boolean}
+ */
+function supportsPixelatedImage() {
+    if(_supportsPixelated !== null) { // only run the feature detection once
+        return _supportsPixelated;
+    }
+    if(Lib.isIE()) {
+        _supportsPixelated = false;
+    } else {
+        var declarations = Array.from(constants.CSS_DECLARATIONS).reverse();
+        var supports = window.CSS && window.CSS.supports || window.supportsCSS;
+        if(typeof supports === 'function') {
+            _supportsPixelated = declarations.some(function(d) {
+                return supports.apply(null, d);
+            });
+        } else {
+            var image3 = Drawing.tester.append('image');
+            var cStyles = window.getComputedStyle(image3.node());
+            image3.attr('style', constants.STYLE);
+            _supportsPixelated = declarations.some(function(d) {
+                var value = d[1];
+                return cStyles.imageRendering === value ||
+                       cStyles.imageRendering === value.toLowerCase();
+            });
+            image3.remove();
+        }
+    }
+    return _supportsPixelated;
+}
+
+module.exports = supportsPixelatedImage;

src/traces/heatmap/calc.js

@@ -90,32 +90,31 @@ module.exports = function calc(gd, trace) {
         Lib.warn('cannot use zsmooth: "fast": ' + msg);
     }
 
-    // check whether we really can smooth (ie all boxes are about the same size)
-    if(zsmooth === 'fast') {
-        if(xa.type === 'log' || ya.type === 'log') {
-            noZsmooth('log axis found');
-        } else if(!isHist) {
-            if(x.length) {
-                var avgdx = (x[x.length - 1] - x[0]) / (x.length - 1);
-                var maxErrX = Math.abs(avgdx / 100);
-                for(i = 0; i < x.length - 1; i++) {
-                    if(Math.abs(x[i + 1] - x[i] - avgdx) > maxErrX) {
-                        noZsmooth('x scale is not linear');
-                        break;
-                    }
-                }
-            }
-            if(y.length && zsmooth === 'fast') {
-                var avgdy = (y[y.length - 1] - y[0]) / (y.length - 1);
-                var maxErrY = Math.abs(avgdy / 100);
-                for(i = 0; i < y.length - 1; i++) {
-                    if(Math.abs(y[i + 1] - y[i] - avgdy) > maxErrY) {
-                        noZsmooth('y scale is not linear');
-                        break;
-                    }
+    function scaleIsLinear(s) {
+        if(s.length > 1) {
+            var avgdx = (s[s.length - 1] - s[0]) / (s.length - 1);
+            var maxErrX = Math.abs(avgdx / 100);
+            for(i = 0; i < s.length - 1; i++) {
+                if(Math.abs(s[i + 1] - s[i] - avgdx) > maxErrX) {
+                    return false;
                 }
             }
         }
+        return true;
+    }
+
+    // Check whether all brick are uniform
+    trace._islinear = false;
+    if(xa.type === 'log' || ya.type === 'log') {
+        if(zsmooth === 'fast') {
+            noZsmooth('log axis found');
+        }
+    } else if(!scaleIsLinear(x)) {
+        if(zsmooth === 'fast') noZsmooth('x scale is not linear');
+    } else if(!scaleIsLinear(y)) {
+        if(zsmooth === 'fast') noZsmooth('y scale is not linear');
+    } else {
+        trace._islinear = true;
     }
 
     // create arrays of brick boundaries, to be used by autorange and heatmap.plot

src/traces/heatmap/plot.js

@@ -15,6 +15,8 @@ var makeColorScaleFuncFromTrace = require('../../components/colorscale').makeCol
 var xmlnsNamespaces = require('../../constants/xmlns_namespaces');
 var alignmentConstants = require('../../constants/alignment');
 var LINE_SPACING = alignmentConstants.LINE_SPACING;
+var supportsPixelatedImage = require('../../lib/supports_pixelated_image');
+var PIXELATED_IMAGE_STYLE = require('../../constants/pixelated_image').STYLE;
 
 var labelClass = 'heatmap-label';
 
@@ -109,11 +111,18 @@ module.exports = function(gd, plotinfo, cdheatmaps, heatmapLayer) {
             y = cd0.yfill;
         }
 
+        var drawingMethod = 'default';
+        if(zsmooth) {
+            drawingMethod = zsmooth === 'best' ? 'smooth' : 'fast';
+        } else if(trace._islinear && xGap === 0 && yGap === 0 && supportsPixelatedImage()) {
+            drawingMethod = 'fast';
+        }
+
         // make an image that goes at most half a screen off either side, to keep
-        // time reasonable when you zoom in. if zsmooth is true/fast, don't worry
+        // time reasonable when you zoom in. if drawingMethod is fast, don't worry
         // about this, because zooming doesn't increase number of pixels
         // if zsmooth is best, don't include anything off screen because it takes too long
-        if(zsmooth !== 'fast') {
+        if(drawingMethod !== 'fast') {
             var extra = zsmooth === 'best' ? 0 : 0.5;
             left = Math.max(-extra * xa._length, left);
             right = Math.min((1 + extra) * xa._length, right);
@@ -127,7 +136,9 @@ module.exports = function(gd, plotinfo, cdheatmaps, heatmapLayer) {
         // setup image nodes
 
         // if image is entirely off-screen, don't even draw it
-        var isOffScreen = (imageWidth <= 0 || imageHeight <= 0);
+        var isOffScreen = (
+            left >= xa._length || right <= 0 || top >= ya._length || bottom <= 0
+        );
 
         if(isOffScreen) {
             var noImage = plotGroup.selectAll('image').data([]);
@@ -140,7 +151,7 @@ module.exports = function(gd, plotinfo, cdheatmaps, heatmapLayer) {
         // generate image data
 
         var canvasW, canvasH;
-        if(zsmooth === 'fast') {
+        if(drawingMethod === 'fast') {
             canvasW = n;
             canvasH = m;
         } else {
@@ -158,7 +169,7 @@ module.exports = function(gd, plotinfo, cdheatmaps, heatmapLayer) {
         // map brick boundaries to image pixels
         var xpx,
             ypx;
-        if(zsmooth === 'fast') {
+        if(drawingMethod === 'fast') {
             xpx = xrev ?
                 function(index) { return n - 1 - index; } :
                 Lib.identity;
@@ -235,7 +246,7 @@ module.exports = function(gd, plotinfo, cdheatmaps, heatmapLayer) {
             return setColor(z00 + xinterp.frac * dx + yinterp.frac * (dy + xinterp.frac * dxy));
         }
 
-        if(zsmooth) { // best or fast, works fastest with imageData
+        if(drawingMethod !== 'default') { // works fastest with imageData
             var pxIndex = 0;
             var pixels;
 
@@ -245,7 +256,7 @@ module.exports = function(gd, plotinfo, cdheatmaps, heatmapLayer) {
                 pixels = new Array(canvasW * canvasH * 4);
             }
 
-            if(zsmooth === 'best') {
+            if(drawingMethod === 'smooth') { // zsmooth="best"
                 var xForPx = xc || x;
                 var yForPx = yc || y;
                 var xPixArray = new Array(xForPx.length);
@@ -273,7 +284,7 @@ module.exports = function(gd, plotinfo, cdheatmaps, heatmapLayer) {
                         putColor(pixels, pxIndex, c);
                     }
                 }
-            } else { // zsmooth = fast
+            } else { // drawingMethod = "fast" (zsmooth = "fast"|false)
                 for(j = 0; j < m; j++) {
                     row = z[j];
                     yb = ypx(j);
@@ -297,7 +308,8 @@ module.exports = function(gd, plotinfo, cdheatmaps, heatmapLayer) {
             }
 
             context.putImageData(imageData, 0, 0);
-        } else { // zsmooth = false -> filling potentially large bricks works fastest with fillRect
+        } else { // rawingMethod = "default" (zsmooth = false)
+            // filling potentially large bricks works fastest with fillRect
             // gaps do not need to be exact integers, but if they *are* we will get
             // cleaner edges by rounding at least one edge
             var xGapLeft = Math.floor(xGap / 2);
@@ -353,6 +365,10 @@ module.exports = function(gd, plotinfo, cdheatmaps, heatmapLayer) {
             'xlink:href': canvas.toDataURL('image/png')
         });
 
+        if(drawingMethod === 'fast' && !zsmooth) {
+            image3.attr('style', PIXELATED_IMAGE_STYLE);
+        }
+
         removeLabels(plotGroup);
 
         var texttemplate = trace.texttemplate;

src/traces/image/constants.js

@@ -47,18 +47,5 @@ module.exports = {
             },
             suffix: ['°', '%', '%', '']
         }
-    },
-    // For pixelated image rendering
-    // http://phrogz.net/tmp/canvas_image_zoom.html
-    // https://developer.mozilla.org/en-US/docs/Web/CSS/image-rendering
-    pixelatedStyle: [
-        'image-rendering: optimizeSpeed',
-        'image-rendering: -moz-crisp-edges',
-        'image-rendering: -o-crisp-edges',
-        'image-rendering: -webkit-optimize-contrast',
-        'image-rendering: optimize-contrast',
-        'image-rendering: crisp-edges',
-        'image-rendering: pixelated',
-        ''
-    ].join('; ')
+    }
 };

src/traces/image/plot.js

@@ -5,21 +5,21 @@ var Lib = require('../../lib');
 var strTranslate = Lib.strTranslate;
 var xmlnsNamespaces = require('../../constants/xmlns_namespaces');
 var constants = require('./constants');
-
-var unsupportedBrowsers = Lib.isIOS() || Lib.isSafari() || Lib.isIE();
+var supportsPixelatedImage = require('../../lib/supports_pixelated_image');
+var PIXELATED_IMAGE_STYLE = require('../../constants/pixelated_image').STYLE;
 
 module.exports = function plot(gd, plotinfo, cdimage, imageLayer) {
     var xa = plotinfo.xaxis;
     var ya = plotinfo.yaxis;
 
-    var supportsPixelatedImage = !(unsupportedBrowsers || gd._context._exportedPlot);
+    var supportsPixelated = !gd._context._exportedPlot && supportsPixelatedImage();
 
     Lib.makeTraceGroups(imageLayer, cdimage, 'im').each(function(cd) {
         var plotGroup = d3.select(this);
         var cd0 = cd[0];
         var trace = cd0.trace;
         var realImage = (
-            ((trace.zsmooth === 'fast') || (trace.zsmooth === false && supportsPixelatedImage)) &&
+            ((trace.zsmooth === 'fast') || (trace.zsmooth === false && supportsPixelated)) &&
             !trace._hasZ && trace._hasSource && xa.type === 'linear' && ya.type === 'linear'
         );
         trace._realImage = realImage;
@@ -131,7 +131,7 @@ module.exports = function plot(gd, plotinfo, cdimage, imageLayer) {
 
         image3.exit().remove();
 
-        var style = (trace.zsmooth === false) ? constants.pixelatedStyle : '';
+        var style = (trace.zsmooth === false) ? PIXELATED_IMAGE_STYLE : '';
 
         if(realImage) {
             var xRange = Lib.simpleMap(xa.range, xa.r2l);

tasks/test_syntax.js

@@ -193,7 +193,7 @@ function assertSrcContents() {
          * - If you use conforms to these rules, you may update
          *   KNOWN_GET_COMPUTED_STYLE_CALLS to count the new use.
          */
-        var KNOWN_GET_COMPUTED_STYLE_CALLS = 6;
+        var KNOWN_GET_COMPUTED_STYLE_CALLS = 7;
         if(getComputedStyleCnt !== KNOWN_GET_COMPUTED_STYLE_CALLS) {
             logs.push('Expected ' + KNOWN_GET_COMPUTED_STYLE_CALLS +
                 ' window.getComputedStyle calls, found ' + getComputedStyleCnt +