Performant Way to create checkerboard pattern - c++

Question

So I have an image that I want to overlay with a checkerboard pattern.
This is what I have come up with so far:
for ( uint_8 nRow = 0; nRow < image.width(); ++nRow)
for (uint_8 nCol = 0; nCol < image.height(); ++nCol)
if(((nRow/20 + nCol/20) % 2) == 0)
memset(&image.data[nCol + nRow], 0, 1);
Produces a white image unfortunately. I dont think this is very performant because memset is called for every single pixel in the image instead of multiple.
Why does this code not produce a chckerboard pattern? How would you improve it?

Answer 1

For better performance, don't treat the image as a 2-dimensional entity. Instead, look at it as a 1D array of continuous data, where all lines of the image are arranged one after the other.
With this approach, you can write the pattern in one go with a single loop, where in every iteration you memset() multiple adjacent pixels and increase the index by twice the amount of pixels you set:
int data_size = image.width() * image.height();
for (auto it = image.data; it < image.data + data_size; it += 20) {
memset(it, 0, 20);
if (((it - data) + 40) % (20 * 400) == 0) {
it += 40;
} else if (((it - data) + 20) % (20 * 400) != 0) {
it += 20;
}
}
(Replace auto with the type of image.data if you're not using C++11; I suspect it's unsigned char*.)
This is quite friendly for the CPU cache prefetch. It's also friendly for the compiler, which can potentially vectorize and/or perform loop unrolling.

Answer 2

If you have an image's dimensions which are multiple of the checker square size :
(I coded in C but it is fairly easy to transpose to C++)
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#define uint unsigned int
#define WIDTH 40
#define HEIGHT 40
#define BLOCK_SIZE 5
void create_checker_row(uint* row, uint size_block, uint nb_col, uint offset )
{
uint ic;
for (ic = size_block*offset ; ic < nb_col; ic+= 2*size_block )
{
memset( (row + ic) , 0, size_block*sizeof(uint) );
}
}
int main()
{
uint ir,ic;
// image creation
uint* pixels = (uint*) malloc(WIDTH*HEIGHT*sizeof(uint));
for (ir = 0; ir < WIDTH; ir++)
{
for ( ic = 0; ic < HEIGHT; ic++)
{
// arbitrary numbers
pixels[ir*WIDTH + ic] = (ir*WIDTH + ic) % 57 ;
printf("%d,", pixels[ir*WIDTH + ic] );
}
printf("\n");
}
for (ir = 0; ir < WIDTH; ir++)
{
create_checker_row( pixels + ir*WIDTH , // pointer at the beggining of n-th row
BLOCK_SIZE , // horizontal length for square
WIDTH , // image width
(ir/BLOCK_SIZE) % 2 // offset to create the checker pattern
);
}
// validation
printf("\n");
printf("Validation \n");
printf("\n");
for (ir = 0; ir < WIDTH; ir++)
{
for ( ic = 0; ic < HEIGHT; ic++)
{
printf("%d,", pixels[ir*WIDTH + ic] );
}
printf("\n");
}
return 0;
}
Seems pretty checkered for me : http://ideone.com/gp9so6

Answer 3

I use this and stb_image_write.h
#include <stdlib.h>
#include <stb_image_write.h>
int main(int argc, char *argv[])
{
const int w = 256, h = 256, ch = 4, segments = 8, box_sz = w / segments;
unsigned char rgba_fg[4] = {255, 255, 0, 255}; //yellow
unsigned char rgba_bg[4] = {255, 0, 0, 255}; //red
unsigned char* data = calloc(w * h * ch, sizeof(unsigned char));
int swap = 0;
int fill = 0; /* set to 1 to fill fg first*/
unsigned char* col = NULL;
for(int i = 0; i < w * h; i++)
{
if(i % (w * box_sz) == 0 && i != 0)
swap = !swap;
if(i % box_sz == 0 && i != 0)
fill = !fill;
if(fill)
{
if(swap)
col = rgba_bg;
else
col = rgba_fg;
}else
{
if(swap)
col = rgba_fg;
else
col = rgba_bg;
}
for(int j = 0; j < ch; j++)
{
data[i*ch + j] = col[j];
}
}
stbi_write_png("checker.png", w, h, ch, data, 0);
free(data);
return 0;
}
Its a bit slow with large images but gets the job done if you cache them