| 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

NSA Interceptng Gmail During Delivery; General IIR Filters; What's happening with Scala?; Interesting (But Largely Illegible) Typeface; Retiring Essentialism; Poorest in UK, Poorest in N Europe; I Want To Be A Redneck!; Reverse Racism; The Lost Art Of Nomography; IBM Data Center (Photo); Interesting Account Of Gamma Hack; The Most Interesting Audiophile In The World; How did the first world war actually end?; Ky - Restaurant Santiago; The Black Dork Lives!; The UN Requires Unaninmous Decisions; LPIR - Steganography in Practice; How I Am 6; Clear Explanation of Verizon / Level 3 / Netflix; Teenage Girls; Formalising NSA Attacks; Switching Brakes (Tektro Hydraulic); Naim NAP 100 (Power Amp); AKG 550 First Impressions; Facebook manipulates emotions (no really); Map Reduce "No Longer Used" At Google; Removing RAID metadata; New Bike (Good Bike Shop, Santiago Chile); Removing APE Tags in Linux; Compiling Python 3.0 With GCC 4.8; Maven is Amazing; Generating Docs from a GitHub Wiki; Modular Shelves; Bash Best Practices; Good Emergency Gasfiter (Santiago, Chile); Readings in Recent Architecture; Roger Casement; Integrated Information Theory (Or Not); Possibly undefined macro AC_ENABLE_SHARED; Update on Charges; Sunburst Visualisation; Spectral Embeddings (Distances -> Coordinates); Introduction to Causality; Filtering To Help Colour-Blindness; ASUS 1015E-DS02 Too; Ready Player One; Writing Clear, Fast Julia Code; List of LatAm Novels; Running (for women); Building a Jenkins Plugin and a Jar (for Command Line use); Headphone Test Recordings; Causal Consistency; The Quest for Randomness; 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

© 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