ttfmp.rb

#!/usr/bin/ruby
#
# Calculate "Time to First Meaningful Paint" from trace events
#   usage: ttfmp.rb trace.json

require 'json'

class TraceAnalyzer
  def initialize(trace)
    @json = JSON.parse(IO.read(trace))
    parse
  end

  def parse
    @layouts = {}  # timestamp -> layout counters
    @paints = []   # timestamps
    mainFrameID = nil

    @json.each do |item|
      args = item['args'] || {}

      case item['name']
      when 'navigationStart'
        unless mainFrameID
          mainFrameID = args['frame']
          @navStartTime = item['ts']
        end

      when 'FrameView::performLayout'
        if args['counters'] && args['frame'] == mainFrameID
          @layouts[item['ts']] = args['counters']
        end

      when 'Paint'
        if args['data'] && args['data']['frame'] == mainFrameID
          @paints.push item['ts']
        end

      when 'firstContentfulPaint'
        if args['frame'] == mainFrameID
          @firstContentfulPaint = item['ts']
        end
      end
    end
  end

  def firstMeaningfulPaint(heuristics = {})
    layoutTime = nil
    maxSoFar = 0
    pending = 0
    @layouts.each do |ts, counters|
      next if counters['host'].empty? || counters['visibleHeight'] == 0

      layouts = counters['LayoutObjectsThatHadNeverHadLayout'] || 0
      if heuristics[:pageHeight]
        significance = layouts / heightRatio(counters)
      else
        significance = layouts
      end

      if heuristics[:webFont] && counters['hasBlankText']
        pending += significance
      else
        significance += pending
        pending = 0
        if significance > maxSoFar
          maxSoFar = significance
          layoutTime = ts
        end
      end
    end
    paintTime = @paints.find {|ts| ts > layoutTime}
    (paintTime - @navStartTime) / 1000
  end

  def firstContentfulPaint
    (@firstContentfulPaint - @navStartTime) / 1000
  end

  private
  def heightRatio(counters)
    ([counters['contentsHeightBefore'].to_f / counters['visibleHeight'], 1].max +
     [counters['contentsHeightAfter'].to_f / counters['visibleHeight'], 1].max) / 2
  end

end

if $0 == __FILE__
  ARGV.each do |fname|
    trace = TraceAnalyzer.new(fname)
    puts({"TTFCP" => trace.firstContentfulPaint,
          "Basic" => trace.firstMeaningfulPaint(),
          "PageHeight" => trace.firstMeaningfulPaint(pageHeight: true),
          "WebFont" => trace.firstMeaningfulPaint(webFont: true),
          "Full" => trace.firstMeaningfulPaint(pageHeight: true, webFont: true)
         }.to_s)
  end
end

Explanation and document: Time to First Meaningful Paint: a layout-based approach

These traces are dependent on new instrumentation a la https://codereview.chromium.org/1773633003