Fix issues with opacity gradients

When there is opacity ≠ 1 specified, the gradient is drawn and masked on a "Tiling Pattern". The tiling has BBox, XStep & YStep values equal to the dimensions of the page, to avoid any clipping or tiling repetition. Patterns are unaffected by the "Current Transformation Matrix" so the clipping rectangle is always aligned with the page borders (SMasks and Groups don't have this property). Problems could possibly happen if a gradient is embedded on multiple pages (this doesn't work anyway) of different size.

This pull request resolves the following issues:
- Fix use of opacity gradients with fillAndStroke (#214)
- Fix the reuse of the same gradient after transformations
- Fix clipping of some gradients with opacity
- Add 'transform' function to the gradient object

lib/gradient.coffee

@@ -2,7 +2,7 @@ class PDFGradient
   constructor: (@doc) ->
     @stops = []
     @embedded = no
-    @transform = [1, 0, 0, 1, 0, 0]
+    @_transform = [1, 0, 0, 1, 0, 0]
     @_colorSpace = 'DeviceRGB'
     
   stop: (pos, color, opacity = 1) ->
@@ -10,9 +10,14 @@ class PDFGradient
     @stops.push [pos, @doc._normalizeColor(color), opacity]
     return this
     
-  embed: ->
-    return if @embedded or @stops.length is 0
+  transform: (m11, m12, m21, m22, dx, dy) ->
+    @_transform = [m11, m12, m21, m22, dx, dy]
+    return this
+
+  embed: (m) ->
+    return if @stops.length is 0
     @embedded = yes
+    @matrix = m
     
     # if the last stop comes before 100%, add a copy at 100%
     last = @stops[@stops.length - 1]
@@ -53,17 +58,6 @@ class PDFGradient
         
     @id = 'Sh' + (++@doc._gradCount)
     
-    # apply gradient transform to existing document ctm
-    m = @doc._ctm.slice()
-    [m0, m1, m2, m3, m4, m5] = m
-    [m11, m12, m21, m22, dx, dy] = @transform
-    m[0] = m0 * m11 + m2 * m12
-    m[1] = m1 * m11 + m3 * m12
-    m[2] = m0 * m21 + m2 * m22
-    m[3] = m1 * m21 + m3 * m22
-    m[4] = m0 * dx + m2 * dy + m4
-    m[5] = m1 * dx + m3 * dy + m5
-
     shader = @shader fn
     shader.end()
     
@@ -71,9 +65,8 @@ class PDFGradient
       Type: 'Pattern'
       PatternType: 2
       Shading: shader
-      Matrix: (+v.toFixed(5) for v in m)
+      Matrix: (+v.toFixed(5) for v in @matrix)
 
-    @doc.page.patterns[@id] = pattern
     pattern.end()
     
     if (@stops.some (stop) -> stop[2] < 1)
@@ -83,59 +76,70 @@ class PDFGradient
       for stop in @stops
         grad.stop stop[0], [stop[2]]
         
-      grad = grad.embed()
-              
-      group = @doc.ref
-        Type: 'Group'
-        S: 'Transparency'
-        CS: 'DeviceGray'
-        
-      group.end()
+      grad = grad.embed(@matrix)
       
-      resources = @doc.ref
-        ProcSet: ['PDF', 'Text', 'ImageB', 'ImageC', 'ImageI']
-        Shading:
-          Sh1: grad.data.Shading
-          
-      resources.end()
+      pageBBox = [0, 0, @doc.page.width, @doc.page.height]
       
       form = @doc.ref
         Type: 'XObject'
         Subtype: 'Form'
         FormType: 1
-        BBox: [0, 0, @doc.page.width, @doc.page.height]
-        Group: group
-        Resources: resources
+        BBox: pageBBox
+        Group:
+          Type: 'Group'
+          S: 'Transparency'
+          CS: 'DeviceGray'
+        Resources:
+          ProcSet: ['PDF', 'Text', 'ImageB', 'ImageC', 'ImageI']
+          Pattern:
+            Sh1: grad
       
-      form.end "/Sh1 sh"
+      form.write "/Pattern cs /Sh1 scn"
+      form.end "#{pageBBox.join(" ")} re f"
       
-      sMask = @doc.ref
-        Type: 'Mask'
-        S: 'Luminosity'
-        G: form
-          
-      sMask.end()
-        
       gstate = @doc.ref
         Type: 'ExtGState'
-        SMask: sMask
+        SMask:
+          Type: 'Mask'
+          S: 'Luminosity'
+          G: form
       
-      @opacity_id = ++@doc._opacityCount
-      name = "Gs#{@opacity_id}"
-      
-      @doc.page.ext_gstates[name] = gstate
       gstate.end()
-    
-    return pattern
       
+      opacityPattern = @doc.ref
+        Type: 'Pattern'
+        PatternType: 1
+        PaintType: 1
+        TilingType: 2
+        BBox: pageBBox
+        XStep: pageBBox[2]
+        YStep: pageBBox[3]
+        Resources:
+          ProcSet: ['PDF', 'Text', 'ImageB', 'ImageC', 'ImageI']
+          Pattern:
+            Sh1 : pattern
+          ExtGState:
+            Gs1 : gstate
+      
+      opacityPattern.write "/Gs1 gs /Pattern cs /Sh1 scn"
+      opacityPattern.end "#{pageBBox.join(" ")} re f"
+      
+      @doc.page.patterns[@id] = opacityPattern
+      
+    else
+      @doc.page.patterns[@id] = pattern
+      
+    return pattern
+
   apply: (op) ->
-    @embed() unless @embedded
+    # apply gradient transform to existing document ctm
+    [m0, m1, m2, m3, m4, m5] = @doc._ctm.slice()
+    [m11, m12, m21, m22, dx, dy] = @_transform
+    m = [m0 * m11 + m2 * m12, m1 * m11 + m3 * m12, m0 * m21 + m2 * m22, m1 * m21 + m3 * m22, m0 * dx + m2 * dy + m4, m1 * dx + m3 * dy + m5]
+
+    @embed(m) unless @embedded and m.join(" ") is @matrix.join(" ")
     @doc.addContent "/#{@id} #{op}"
-    
-    if @opacity_id
-      @doc.addContent "/Gs#{@opacity_id} gs"
-      @doc._sMasked = true
-    
+
 class PDFLinearGradient extends PDFGradient
   constructor: (@doc, @x1, @y1, @x2, @y2) ->
     super
@@ -166,4 +170,4 @@ class PDFRadialGradient extends PDFGradient
   opacityGradient: ->
     return new PDFRadialGradient(@doc, @x1, @y1, @r1, @x2, @y2, @r2)
       
-module.exports = {PDFGradient, PDFLinearGradient, PDFRadialGradient}
+module.exports = {PDFGradient, PDFLinearGradient, PDFRadialGradient}