Improve speed of image processing

Programming Questions

pierrepas..

Improve speed of image processing

in Programming Questions • 4 months ago

Hello,

I've written a program that grayscales pixels depending on the colour of the neighbouring pixels. I only want to grayscale the pixel if it is white so the first step is to scan through the image pixel by pixel to determine the colour of the pixel. If the pixel is white then the location of the pixel is passed to a method that scans the neighbouring pixels and counts the number of neighbouring white pixels. Based on how many neighbouring pixels are white the colour of the center pixel is then altered.

I want to use this program to process thousands of images (up to 9999), however as the images are processed the program becomes increasingly slow. Do you have any tips as to how I can improve the speed of the image processing?

Thanks

//Define PImage variables
PImage source;
PImage destination;
String baseName = "MRobot_";
boolean more;
int z=1;
int even = 2 ;
void setup()
{
size (1280,800);
}
void draw()
{
do
{
for (z=1; z<9999; z++)
{
//Load source image
source = loadImage(baseName+nf(z,4)+".png");
more = new File(baseName+nf(z,4)+".png").exists();
destination = createImage(source.width, source.height, ALPHA);
//Look at both image's pixels
source.loadPixels();
destination.loadPixels();
//Set the size of the scanning matrix
int matrixsize = 3;
//Loop through every pixel column
for ( int x = 0; x < source.width; x++)
{
//Loop through every pixel row
for (int y = 0; y < source.height; y++)
{
//Calculate 1D pixel location
int loc = x + y * source.width;
//Determine colour of current pixel
color pix = source.pixels[loc];
//If current pixel colour is white examine if the colour needs changing the colour
if ( pix == color(255))
{
//Pass the x y coordinates and matrix info to the greyscale method
//greyscale method then scans the surronding pixels and returns a new colour
color newPix = greyscale(x,y,matrixsize,source);
//Set pixel colour to new value
destination.pixels[loc] = newPix;
}
else
{
destination.pixels[loc] = color(0); //black
}
}
}
//Update pixels in destination
destination.updatePixels();
//Set background to black
background(0);
//Display the destination image
image(destination,0,0);
for (even=2; even<z*2; even = even +2)
{
//Save image with even number
save(baseName+nf(even,4)+".png");
}
}
}while(more);
}
//Method to greyscale a pixel based on the surronding pixel colours
color greyscale(int x, int y, int matrixsize, PImage img)
{
int greyvalue = 0;
int offset = matrixsize / 2;
// Loop through scanning matrix
for (int i = 0; i < matrixsize; i++)
{
for (int j= 0; j < matrixsize; j++)
{
// What pixel are we testing
int xloc = x+i-offset;
int yloc = y+j-offset;
int loc = xloc + img.width*yloc;
// Make sure we have not walked off the edge of the pixel array
loc = constrain(loc,0,img.pixels.length-1);
//Colour of the current pixel
color pix = img.pixels[loc];
//if the colour of the current pixel is white
// add one to "greyvalue"
if (pix == color(255))
{
greyvalue = greyvalue + 1;
}
else
{
greyvalue = greyvalue + 0;
}
}
}
//Means that there is a least one white pixel within the scanning matrix
if(greyvalue > 0)
{
//Measn there is at least one black pixel within the scanning matrix
if (greyvalue < 9)
{
greyvalue = 255; //white
}
else
{
greyvalue = 100; //grey
}
}
else
{
greyvalue = 0; //black
}
// Make sure colour is within range
greyvalue = constrain(greyvalue,0,255);
// Return the resulting color
return color(greyvalue);
}

Replies(3)

PhiLho

Re: Improve speed of image processing

4 months ago

Random remarks:
- Why is this code in draw()? Obviously, that's batch processing, no you don't plan to repeat the operation, are you?
All this code can live in setup(). Or use noLoop(), at least.
- While the loop on more, when you already have a loop on the predefined number of files?
Moreover, you check the existence of the file after you loaded it! And even if it doesn't exists, you continue the current iteration body... Better do the test before hand, and use break to exist the for loop, if needed.
- Minor optimization (probably not doing anything, but not hurting either): when you use color(0) or color(255), you make a function call, which isn't trivial. Better make two global constants (BLACK and WHITE, for example) and use them.
- You can save destination directly, no need to display it, unless you want a visual feedback, of course. No need to call background() before.
- " as the images are processed the program becomes increasingly slow." How do you know? Visual feedback is slowing?
Note: better use 2.0b9, it solves an issue with image() usage with lot of different images: memory was not released, so it can crash your program, or at least slow it down.

pierrepas..

Re: Improve speed of image processing

4 months ago

Hi PhiLho,

Thanks for taking the time to look at my code.

To answer your questions:

The code was in draw() because it was based on an example I had seen. I have now moved it to setup() as you are right I only need to do the operation once.

The reason for the "while the loop on more" is that there could be any number of files, but as you pointed out it doesn't quite work at the moment! I will move the check of the existence of the file to before I load it and add a break() clause. I will also change the for loop to

for (z=1, z++)

so that the program will work with any number of files.

Thanks for pointing out the minor optimisations. I have implemented them. The only one I can't get to work is removing the visual feedback as when I do this the saved image is blank. Any suggestions?

I could tell the program slowed down as I was watching the files being created in Finder. I also noticed that as the files were saved the program would loop through from the beginning for each file! So it was re-saving every previous image as it processed the next image. This it what was slowing it down.

I realised that the issue was in fact with how I was renaming the files to be saved. The for loop at line 70 was slowing everything down, so i removed it and added a simple counter instead.

even = even + 2;

The code now works much faster with all these changes. It processes over a thousand images in a couple of minutes which is fast enough for me.

PhiLho

Re: Improve speed of image processing

4 months ago

Visual feedback: keep it since it is fast enough now. But for the record, you can do:
destination.save(baseName+nf(even,4)+".png");
directly instead of drawing the image on the sketch and using the sketch's save().

And, indeed, I haven't looked at the file names you used, I supposed you saved the new files elsewhere or with different names, to avoid conflicts...

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