About

Adobe® Photoshop® has a variety of helpful blend modes for compositing images from multiple RGBA layers. This small library provides the same functionality for HTML Canvas Contexts, with the goal of producing the same results as Photoshop.

Syntax

overContext.blendOnto( underContext, blendMode, offsetOptions );
  - overContext   : A CanvasRenderingContext2D
  - underContext  : A CanvasRenderingContext2D
  - blendMode     : A string with the blend mode to use, e.g. 'screen'
  - offsetOptions : [optional] JS Object with some/all of the following keys:
      destX, destY
      The X/Y location in the 'underContext' to blend onto; both default to 0.
      
      sourceX, sourceY
      The X/Y location in the 'overContext' to blend from; both default to 0.
      
      width,height
      The size of the box to blend; both default to 'auto', blending up to the
      right and bottom edges of the 'over' context.
      
      Width and height may less than specified if there is not enough space
      on the over or under contexts to fit the blend.

Use

// Likely an 'offscreen' (not in the DOM) canvas
var over = someCanvas.getContext('2d'); 

// Usually a canvas that is shown on the page
var under = anotherCanvas.getContext('2d');

// Blend all of 'over' onto 'under', starting at the upper left corner
over.blendOnto(under,'screen');

// Blend all of 'over' onto 'under' (again), starting at 17,42 in 'under'
over.blendOnto(under,'multiply',{destX:17,destY:42});

// Blend a 16x16 tile from 'over' onto 'under' (again), starting at 17,42 in 'under'
over.blendOnto(under,'add',{destX:17,destY:42,sourceX:32,sourceY:128,width:16,height:16});

Supported Blend Modes

The following blend modes work perfectly (or as nearly as the vagaries of the HTML Canvas allow):

• normal (or src-over)
• screen
• multiply
• difference

These blend modes mostly work as intended, but have issues when it comes to dealing with the alpha channel:

• exclusion - very subtle color differences (slightly too bright) under limited circumstances.

• src-in - the output of this blend mode is slightly different from the effect of applying the transparency of one layer as a mask to another; the difference only appears in low-opacity areas, however.

  • 

• add (or plus) - Photoshop's "Linear Dodge (add)" blend mode does not perform addition on the opacities of the two layers. I have not yet figured out what it does instead. For now, this mode performs simple numeric addition, the same as the SVG 1.2 "plus" mode.

  • 

• lighten (or lighter) - the result is slightly too dark when the opacity falls and incorrectly 'favors' a higher-opacity source.

  • 

• darken (or darker) - the result is too dark when combining low-opacity regions, and does not properly 'favor' the higher-opacity source.

  • 

• overlay - this is only correct where both the over and under images are 100% opaque; the lower the alpha of either/both images, the more the colors get clamped, resulting in high contrast.

  • 

• hardlight - this is the opposite of "overlay" and experiences similar problems when either image is not fully opaque.

  • 

• colordodge (or dodge) - works correctly only under 100% opacity

  • 

• colorburn (or burn) - works correctly only under 100% opacity

  • 

Requirements/Browser Support

Tested on Safari v5.0, Chrome v8, and FF v3.6. Should work on any user agent that supplies a CanvasRenderingContext2D along with getImageData and putImageData.

About

This library was created around the need solely for a one-off 'screen' blend mode to match the company-mandated style for bar graphs used internally, previously only available via a Microsoft® Excel® template. Clearly this functionality is useful in more contexts than just my one-off, so I decided to make a framework around it and encourage others to help figure out the formulae. Please, fork this project, add blend modes and/or fix math, and send me pull requests! I feel certain that the resources must exist out there on the equations Photoshop uses in the presence of alpha, but so far I have not found them.

Reference Material

PDF Blend Modes: Addendum (January 23, 2006) PDF
SVG Compositing 1.2, Part 1: Primer
Custom blend modes for Flash 10 blog post
Blend Modes in Delphi blog post

License

This library is released under an MIT-style license. That generally means that you are free to do almost anything you want with it as long as you give a bit of credit where credit is due. See the LICENSE file included for the actual legal limitations.