Extraction of specific image pixels as bytes

Question

I have a large set (>10000) of uncompressed 8 bit images. Their size is 1600*1200 pixels.

The first 24 pixels in the upper most line starting from left contain some important information (time stamp etc.) about the image.

I want to read out all byte values of these 24 pixels.

Finally I want to save all byte data as right aligned numbers into a text file, a single line with 24 numbers for each image.

I CANNOT READ IN ALL IMAGES INTO THE MEMORY AT ONCE BECAUSE OF MEMORY PROBLEMS.

For testing I have uploaded a set of 1000 cropped images, each having a dimension of 50*50 pixels. They can be downloaded from here: https://drive.google.com/open?id=0B9wKP6yNcpyfUUlsQU1iVWJ0b2M

How can I improve (speed up) my code?

ChoiceDialog[{FileNameSetter[Dynamic[imageDir], "Directory"], Dynamic[imageDir]}];
SetDirectory[imageDir];

fNames = FileNames["*.png"];
n = Length[fNames];

timeStampValues = Array[0 &, {n, 24}];
SetSharedVariable[timeStampValues];

ParallelDo[

   byteValues = Import[fNames[[i]], "Data"];
   timeStampValues[[i, All]] = Flatten@byteValues[[1, 1 ;; 24]];

   , {i, 1, n}
]; // AbsoluteTiming

{17.7253, Null}

Answer 1

• Start by replacing Import with

  ImageData@First@Image`ImportExportDump`ImageReadPNG[fNames[[‌​i]]]
  

  This is the low level function called by Import on PNG files; calling it directly bypasses A TON of behind-the-scenes checking. On a 350-picture subset, the ParallelDo[Import...] alone was taking 70s on my laptop; the ImageReadPNG version took 4s.

• The Flatten call within the ParallelDo loop also doesn't seem necessary, since byteValues[[1, 1;;24]] is already a flat vector. That won't probably affect timing quite as much though.

In short, I propose to use:

ParallelDo[
  byteValues = ImageData@First@Image`ImportExportDump`ImageReadPNG[fNames[[i]]];
  timeStampValues[[i]] = byteValues[[1, 1 ;; 24]],
  {i, 1, n}]

On your sample data set, I get the following timings:

• your version: 61 seconds
• my version: 9.2 seconds