# Median Filtering 3*3 Borders problem

Hi !

I am developing a 2D median filter (3 * 3) whose code is given below. I thought I managed the edges but apparently not correctly because the image has black bars.

But I can not see what is wrong with writing my kernel …

``````#define BLOCK_X 16

#define BLOCK_Y 16

texture<unsigned char> imgTex;

__global__ void Kernel(unsigned char *d_out, int nx, int ny)

{

// Prepare guards for later accesses

// Guards : at edge ?

bool is_x_top = (tx == 0), is_x_bot = (tx == BLOCK_X - 1);

bool is_y_top = (ty == 0), is_y_bot = (ty == BLOCK_Y - 1);

__shared__ unsigned char smem[BLOCK_X + 2][BLOCK_Y + 2];

// Zero out shared memory (zero padding)

if (is_x_top)		   SMEM(tx-1, ty  ) = 0;

else if (is_x_bot)	  SMEM(tx+1, ty  ) = 0;

if (is_y_top)

{						SMEM(tx  , ty-1) = 0;

if (is_x_top)	   SMEM(tx-1, ty-1) = 0;

else if (is_x_bot)  SMEM(tx+1, ty-1) = 0;

}

else if (is_y_bot)

{						SMEM(tx,   ty+1) = 0;

if (is_x_top)	   SMEM(tx-1, ty+1) = 0;

else if (is_x_bot)  SMEM(tx+1, ty+1) = 0;

}

int x = blockIdx.x * blockDim.x + tx;

int y = blockIdx.y * blockDim.y + ty;

is_x_top &= (x > 0); is_x_bot &= (x < (nx - 1));

is_y_top &= (y > 0); is_y_bot &= (y < (ny - 1));

// Each thread pulls in it's own pixel

SMEM(tx, ty) = IN(x, y);

if (is_x_top)		   SMEM(tx - 1, ty  )   = IN(x - 1, y  );

else if (is_x_bot)	  SMEM(tx + 1, ty  )   = IN(x + 1, y  );

if (is_y_top)

{						SMEM(tx	, ty - 1) = IN(x	, y - 1);

if (is_x_top)	   SMEM(tx - 1, ty - 1) = IN(x - 1, y - 1);

else if (is_x_bot)  SMEM(tx + 1, ty - 1) = IN(x + 1, y - 1);

}

else if (is_y_bot)

{						SMEM(tx	, ty + 1) = IN(x	, y + 1);

if (is_x_top)	   SMEM(tx - 1, ty + 1) = IN(x - 1, y + 1);

else if (is_x_bot)  SMEM(tx + 1, ty + 1) = IN(x + 1, y + 1);

}

unsigned char v[9] = { SMEM(tx - 1, ty - 1), SMEM(tx  , ty - 1), SMEM(tx + 1, ty - 1),

SMEM(tx - 1, ty	), SMEM(tx  , ty	), SMEM(tx + 1, ty	),

SMEM(tx - 1, ty + 1), SMEM(tx  , ty + 1), SMEM(tx + 1, ty + 1) };

for (int i = 0; i < 9; i++)

{

for (int j = i + 1; j < 9; j++)

{

if (v[i] > v[j])

{

unsigned char tmp = v[i];

v[i] = v[j];

v[j] = tmp;

}

}

}

d_out[y * nx + x] = v[4];

}
``````

The texture is 1D, binded with linear memory.

Nobody has an idea ? I really need to understand what’s wrong with that simple function :unsure:

Nobody has an idea ? I really need to understand what’s wrong with that simple function :unsure:

Nobody has an idea ? I really need to understand what’s wrong with that simple function :unsure:

By just looking at the kernel source it’s not always obvious what is wrong.

You do not provide complete repro case because when there is no host code given (how the kernel is called, and a basic input data set). So it’s hard for people to just try the code.

Christian

By just looking at the kernel source it’s not always obvious what is wrong.

You do not provide complete repro case because when there is no host code given (how the kernel is called, and a basic input data set). So it’s hard for people to just try the code.

Christian

By just looking at the kernel source it’s not always obvious what is wrong.

You do not provide complete repro case because when there is no host code given (how the kernel is called, and a basic input data set). So it’s hard for people to just try the code.

Christian

In an attachment, so there are source files: the main, the kernel code and launch, a file to read .Bmp. I work on Windows 7, VS but the code can be run under Linux.
median.zip (7.42 KB)

In an attachment, so there are source files: the main, the kernel code and launch, a file to read .Bmp. I work on Windows 7, VS but the code can be run under Linux.

In an attachment, so there are source files: the main, the kernel code and launch, a file to read .Bmp. I work on Windows 7, VS but the code can be run under Linux.

It seems that you do not check that x and y are valid for d_out

Anyway, I have the impressiong tha you do not use the shared memory in the right way. The right way would be to fiil it, and then apply the filter for pixel not at the border of the shared memory. This way there is only one load per pixel.

Yves

It seems that you do not check that x and y are valid for d_out

Anyway, I have the impressiong tha you do not use the shared memory in the right way. The right way would be to fiil it, and then apply the filter for pixel not at the border of the shared memory. This way there is only one load per pixel.

Yves