| Andrew Cooke | Contents | Latest | RSS | Twitter | Previous | Next

C[omp]ute

Welcome to my blog, which was once a mailing list of the same name and is still generated by mail. Please reply via the "comment" links.

Always interested in offers/projects/new ideas. Eclectic experience in fields like: numerical computing; Python web; Java enterprise; functional languages; GPGPU; SQL databases; etc. Based in Santiago, Chile; telecommute worldwide. CV; email.

Personal Projects

Lepl parser for Python.

Colorless Green.

Photography around Santiago.

SVG experiment.

Professional Portfolio

Calibration of seismometers.

Data access via web services.

Cache rewrite.

Extending OpenSSH.

Last 100 entries

Chat Wars; Real-life Financial Co Without ACID Database...; Flexible Muscle-Based Locomotion for Bipedal Creatures; SQL Performance Explained; The Little Manual of API Design; Multiple Word Sizes; CRC - Next Steps; FizzBuzz; Update on CRCs; Decent Links / Discussion Community; Automated Reasoning About LLVM Optimizations and Undefined Behavior; A Painless Guide To CRC Error Detection Algorithms; Tests in Julia; Dave Eggers: what's so funny about peace, love and Starship?; Cello - High Level C Programming; autoreconf needs tar; Will Self Goes To Heathrow; Top 5 BioInformatics Papers; Vasovagal Response; Good Food in Vina; Chilean Drug Criminals Use Subsitution Cipher; Adrenaline; Stiglitz on the Impact of Technology; Why Not; How I Am 5; Lenovo X240 OpenSuse 13.1; NSA and GCHQ - Psychological Trolls; Finite Fields in Julia (Defining Your Own Number Type); Julian Assange; Starting Qemu on OpenSuse; Noisy GAs/TMs; Venezuela; Reinstalling GRUB with EFI; Instructions For Disabling KDE Indexing; Evolving Speakers; Changing Salt Size in Simple Crypt 3.0.0; Logarithmic Map (Moved); More Info; Words Found in Voynich Manuscript; An Inventory Of 3D Space-Filling Curves; Foxes Using Magnetic Fields To Hunt; 5 Rounds RC5 No Rotation; JP Morgan and Madoff; Ori - Secure, Distributed File System; Physical Unclonable Functions (PUFs); Prejudice on Reddit; Recursion OK; Optimizing Julia Code; Cash Handouts in Brazil; Couple Nice Music Videos; It Also Works!; Adaptive Plaintext; It Works!; RC5 Without Rotation (2); 8 Years...; Attack Against Encrypted Linux Disks; Pushing Back On NSA At IETF; Summary of Experimental Ethics; Very Good Talk On Security, Snowden; Locusts are Grasshoppers!; Vagrant (OpenSuse and IDEs); Interesting Take On Mandela's Context; Haskell Cabal O(n^2) / O(n) Fix; How I Am 4; Chilean Charity Supporting Women; Doing SSH right; Festival of Urban Intervention; Neat Idea - Wormholes Provide Entanglement; And a Link....; Simple Encryption for Python 2.7; OpenSuse 13.1 Is Better!; Little Gain...; More Details on Technofull Damage; Palmrest Cracked Too....; Tecnofull (Lenovo Support) Is Fucking Useless; The Neuroscientist Who Discovered He Was a Psychopath; Interpolating Polynomials; Bottlehead Crack as Pre-amp; Ooops K702!; Bottlehead Crack, AKG K701; Breaking RC5 Without Rotation; Great post thank you; Big Balls of Mud; Phabricator - Tools for working together; Amazing Julia RC5 Code Parameterized By Word Size; Chi-Square Can Be Two-Sided; Why Do Brits Accept Surveillance?; Statistics Done Wrong; Mesas Trape from Bravo; European Report on Crypto Primitives and Protocols; Interesting Omissions; Oryx And Crake (Margaret Atwood); Music and Theory; My Arduino Programs; Elliptic Curve Crypto; Re: Licensing Interpreted Code; Licensing Interpreted Code; ASUS 1015E-DS03 OpenSuse 12.3 SSD; translating lettuce feature files into stub steps files; Re: translating lettuce feature files into stub steps files; A Tale of Two Psychiatrists

© 2006-2013 Andrew Cooke (site) / post authors (content).

Example of Writing Indexed PNG Using PNGJ + Clojure

From: andrew cooke <andrew@...>

Date: Sat, 28 Apr 2012 15:46:42 -0300

Throwing this out there because pngj is a nice little library, but the
examples are fairly opaque and it assumes you know PNG internals.

I'll describe the code (below) and then give a summary.  You may not need the
code to understand how to use the library, so feel free to skip to the
numbered list...


OK, first the code: The function "print-png-indexed" takes an output stream
and returns two functions.

The second function is used to convert each pixel in the image (originally
HSL) into an index.  This palette of colours that this index references is the
"colours" atom in "print-png-indexed".

Once the first function takes a set of nested lists of index values (orginally
these would have been pixels, but they will have been converted by the second
function - I hope you are following!) and writes them to the output stream,
using the pngj library (after first writing the palette).


You don't have to understand all the above to use this, I hope.  What is
important is the process:

 1 - change pixels to indices, building up a colour-index map as you go.

 2 - start up the pngj library, with ImageInfo and PngWriter

 3 - use the colour-index map to populate a PngChunkPLTE instance.

 4 - write the PngChunkPLTE palette via the image metadata.

 5 - write the indices via the ImageLine.

Andrew


  (defn print-png-indexed [os]
    (let [colours (atom {})]
      [(fn [size rows]
	 ; 8 bits, no alpha, no grayscale, indexed
	 (let [info (ImageInfo. size size 8 Boolean/FALSE Boolean/FALSE Boolean/TRUE)
	       writer (PngWriter. os info)
	       line (ImageLine. info)
	       palette (PngChunkPLTE. info)]
	   (.setNentries palette (count @colours))
	   (doseq [rgb (keys @colours)]
	     (let [[r g b] rgb]
	       (.setEntry palette (@colours rgb) r g b)))
	   (.queueChunk (.getMetadata writer) palette)
	   (doseq [[i row] (map-indexed vector rows)]
  ;           (println row)
	     (.setScanLine line (int-array row))
	     (.writeRow writer line i))
	   (.end writer)))
       (fn [hsl]
	 (let [rgb (hsl-to-8-bit hsl)]
	   (when (nil? (@colours rgb))
	     (swap! colours #(assoc % rgb (count %))))
	   (@colours rgb)))]))

Comment on this post