Modify the webgl line feature shaders.

Chrome 65 and 66 when running in xvfb with openmesa do not compute
atan(0, <negative>) correctly.  Also, there are occasional int() casts
that round down even when they are small integers.  These changes make
image comparisons more robust by working around the atan issue (this may
be the issue detailed in
chromium.googlesource.com/angle/angle/commit/da9fb09).  I have submitted
a new issue to chromium:
https://bugs.chromium.org/p/chromium/issues/detail?id=841296 to track
this problem.

With this commit, the baselines were regenerated with Chrome 64, and the
test strictness requirements changed in PR #792 have been reverted.
There are subtly differences between Chrome 64 and 66 (font aliasing is
slightly different, and I think the other changes are because the shader
float precision is subtly different, though I'm not sure of that).

src/gl/lineFeature.js

@@ -109,17 +109,24 @@ var gl_lineFeature = function (arg) {
           'const float PI = 3.14159265358979323846264;',
 
           'vec4 viewCoord(vec3 c) {',
-          '  vec4 result = projectionMatrix * modelViewMatrix * vec4(c.xyz, 1);',
+          '  vec4 result = projectionMatrix * modelViewMatrix * vec4(c.xyz, 1.0);',
           '  if (result.w != 0.0)  result = result / result.w;',
           '  return result;',
           '}',
 
+          'float atan2(float y, float x) {',
+          '  if (x > 0.0)  return atan(y / x);',
+          '  if (x < 0.0 && y >= 0.0)  return atan(y / x) + PI;',
+          '  if (x < 0.0)  return atan(y / x) - PI;',
+          '  return sign(y) * 0.5 * PI;',
+          '}',
+
           'void main(void)',
           '{',
           /* If any vertex has been deliberately set to a negative opacity,
            * skip doing computations on it. */
           '  if (strokeOpacity < 0.0) {',
-          '    gl_Position = vec4(2, 2, 0, 1);',
+          '    gl_Position = vec4(2.0, 2.0, 0.0, 1.0);',
           '    return;',
           '  }',
           /* convert coordinates.  We have four values, since we need to
@@ -133,10 +140,10 @@ var gl_lineFeature = function (arg) {
           // calculate line segment vector and angle
           '  vec2 deltaCB = C.xy - B.xy;',
           '  if (deltaCB == vec2(0.0, 0.0)) {',
-          '    gl_Position = vec4(2, 2, 0, 1);',
+          '    gl_Position = vec4(2.0, 2.0, 0.0, 1.0);',
           '    return;',
           '  }',
-          '  float angleCB = atan(deltaCB.y / aspect, deltaCB.x);',
+          '  float angleCB = atan2(deltaCB.y / aspect, deltaCB.x);',
           // values we need to pass along
           '  strokeColorVar = vec4(strokeColor, strokeOpacity);',
           // extract values from our flags field
@@ -169,10 +176,10 @@ var gl_lineFeature = function (arg) {
           // offset, then the functional join angle is not simply half the
           // angle between the two lines, but rather half the angle of the
           // inside edge of the the two lines.
-          '  float cosABC, sinABC, cosBCD, sinBCD;',  // of half angles
+          '  float cosABC = 1.0, sinABC = 0.0, cosBCD = 1.0, sinBCD = 0.0;',  // of half angles
           // handle near end
           '  if (nearMode >= 4) {',
-          '    float angleBA = atan((B.y - A.y) / aspect, B.x - A.x);',
+          '    float angleBA = atan2((B.y - A.y) / aspect, B.x - A.x);',
           '    if (A.xy == B.xy)  angleBA = angleCB;',
           '    float angleABC = angleCB - angleBA;',
           // ensure angle is in the range [-PI, PI], then take the half angle
@@ -205,7 +212,7 @@ var gl_lineFeature = function (arg) {
 
           // handle far end
           '  if (farMode >= 4) {',
-          '    float angleDC = atan((D.y - C.y) / aspect, D.x - C.x);',
+          '    float angleDC = atan2((D.y - C.y) / aspect, D.x - C.x);',
           '    if (D.xy == C.xy)  angleDC = angleCB;',
           '    float angleBCD = angleDC - angleCB;',
           // ensure angle is in the range [-PI, PI], then take the half angle
@@ -231,7 +238,7 @@ var gl_lineFeature = function (arg) {
           '  gl_Position = vec4(',
           '    B.x + (xOffset * cos(angleCB) - yOffset * sin(angleCB)) * pixelWidth,',
           '    B.y + (xOffset * sin(angleCB) + yOffset * cos(angleCB)) * pixelWidth * aspect,',
-          '    B.z, 1);',
+          '    B.z, 1.0);',
           // store other values needed to determine which pixels to plot.
           '  float lineLength = length(vec2(deltaCB.x, deltaCB.y / aspect)) / pixelWidth;',
 
@@ -262,7 +269,11 @@ var gl_lineFeature = function (arg) {
   function createFragmentShader(allowDebug) {
     var fragmentShaderSource = [
           '#ifdef GL_ES',
-          '  precision highp float;',
+          '  #ifdef GL_FRAGMENT_PRECISION_HIGH',
+          '    precision highp float;',
+          '  #else',
+          '    precision mediump float;',
+          '  #endif',
           '#endif',
           'varying vec4 strokeColorVar;',
           'varying vec4 subpos;',
@@ -275,8 +286,8 @@ var gl_lineFeature = function (arg) {
           '  vec4 color = strokeColorVar;',
           allowDebug ? '  bool debug = bool(mod(fixedFlags, 2.0));' : '',
           '  float opacity = 1.0;',
-          '  int nearMode = int(info.x);',
-          '  int farMode = int(info.y);',
+          '  int nearMode = int(floor(info.x + 0.5));',
+          '  int farMode = int(floor(info.y + 0.5));',
           '  float cosABC = angles.x;',
           '  float sinABC = angles.y;',
           '  float cosBCD = angles.z;',

testing/test-data/base-images.tgz.md5

@@ -1 +1 @@
-2333a69e96ce063d10862bd8d563eb3d
+24a2ffe0807138fe13c33ddeccb2b426

testing/test-data/base-images.tgz.url

@@ -1 +1 @@
-https://data.kitware.com/api/v1/file/5a9ed5838d777f0685786207/download
+https://data.kitware.com/api/v1/file/5af1fcff8d777f0685797d19/download

tests/gl-cases/glPolygons.js

@@ -2446,9 +2446,7 @@ describe('glPolygons', function () {
       layer: layer
     }).read(JSON.stringify(data), function () {
       myMap.draw();
-      // maximum error is 0.0030 because of a change in Chrome 65.  When new
-      // versions are released, see if this can be moved back to 0.0015
-      imageTest.imageTest('glMultiPolygons', null, 0.0030, done, myMap.onIdle, 0, 2);
+      imageTest.imageTest('glMultiPolygons', null, 0.0015, done, myMap.onIdle, 0, 2);
     });
   });
 

tests/headed-cases/lines.js

@@ -16,9 +16,7 @@ describe('lines example', function () {
   it('more lines', function (done) {
     base$ = $('iframe#map')[0].contentWindow.jQuery;
     base$('#lines').val(100000).trigger('change');
-    // maximum error is 0.0030 because of a change in Chrome 65.  When new
-    // versions are released, see if this can be moved back to 0.0015
-    imageTest.imageTest('exampleLines100k', '#map', 0.0030, done, null, 0, 2, '#map.ready[segments="100000"]');
+    imageTest.imageTest('exampleLines100k', '#map', 0.0015, done, null, 0, 2, '#map.ready[segments="100000"]');
   }, 10000);
   it('thin preset', function (done) {
     base$('button.preset').eq(1).trigger('click');