Wrong gradient attributes occasionally being rendered

[jacksop]

I have a document with several small polygon shapes in different colours over many pages. I am experiencing an issue where occasionally a shape is rendered in the wrong colour.

I believe this is caused by collisions in the registry key used to store the gradient as a page resource. The key is created from a digest of the gradient attributes, packed as unsigned 8-bit characters.

Example

This small example illustrates the issue.

Prawn::Document.generate('gradient.pdf') do                                        
  stroke_axis                                                                      
                                                                                   
  # top left solid pink                                                            
  fill_color 'fe00ff'                                                              
  fill_rectangle [150,512], 100, 100                                               
                                                                                   
  # top right gradient pink                                                        
  fill_gradient [256,512], [356,512], 'ffffff', 'fe00ff'                           
  fill_rectangle [256,512], 100, 100                                               
                                                                                   
  # bottom left solid blue                                                         
  fill_color '0000ff'                                                              
  fill_rectangle [150,256], 100, 100                                               
                                                                                   
  # bottom right gradient blue                                                     
  fill_gradient [256,256], [356,256], 'ffffff', '0000ff'                           
  fill_rectangle [256,256], 100, 100                                                                                                                           
end

The key for the gradient on the bottom right is identical to the previously generated key for the top right gradient and consequently the rectangle uses the old gradient definition.

Solution

Would it be possible to avoid packing the attributes down to 8-bit values and to also use a SHA2 digest (for a longer key)?

Suggest changing the method in lib/prawn/graphics/patterns.rb from:

def gradient_registry_key(gradient)                                                
  _x1, _y1, x2, y2, transformation = gradient_coordinates(gradient)                
                                                                                   
  key = [                                                                          
    gradient.type.to_s,                                                            
    transformation,                                                                
    x2, y2,                                                                        
    gradient.r1 || -1, gradient.r2 || -1,                                          
    gradient.stops.length,                                                         
    gradient.stops.map { |s| [s.position, s.color] }                               
  ].flatten                                                                        
  Digest::SHA1.hexdigest(key.pack('HC*'))                                             
end

to:

def gradient_registry_key(gradient)                                                
  _x1, _y1, x2, y2, transformation = gradient_coordinates(gradient)                
                                                                                   
  key = [                                                                          
    gradient.type.to_s,                                                            
    transformation,                                                                
    x2, y2,                                                                        
    gradient.r1 || -1, gradient.r2 || -1,                                          
    gradient.stops.length,                                                         
    gradient.stops.map { |s| [s.position, s.color] }                               
  ].flatten                                                                        
  Digest::SHA2.hexdigest key.join(',')                                             
end

[cervinka]

I have similar problem. Suggested solution from @jacksop works for me.

[pointlessone]

@jacksop Thank you for reporting this.

Sorry for the delay. I’m a bit busy right now.

A quick check in the code indeed reveals shortcomings of key construction. Hashing function is OK. We don’t really need longer keys. The flaw is in the pack. Just switching to join is enough to eliminate the issue.

As I mentioned, I’m a bit busy right now so I wouldn’t be able to fix this fast. I’d gladly accept a PR though. So if you wish to work on this feel free to take it. I can help with an advice.

[jacksop]

Thanks @pointlessone. I'll put together a PR.

[jacksop]

@pointlessone Do I need specific access in order to push a branch?

[pointlessone]

@jacksop Yes. Try submitting a PR from your fork.