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Several small improvements #3

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Several small improvements #3

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05c4587
Comma separated file for Gnuplot
Jan 12, 2020
0a01bcd
Add variables for grid size
Jan 12, 2020
ac42708
Averaging over the entire grid cell
Jan 12, 2020
a889235
Read sensor model from the RAW header and set appropriate black level
Jan 12, 2020
0f526c7
Lens shading table binary output
Jan 12, 2020
423977f
Search the brightest area in the image
hasepompase Jan 21, 2020
c0f3877
fix indentation
Jan 21, 2020
b2a950e
Changed calculation of grid_width and grid_height to initial method
Jan 21, 2020
f290689
Removed double calculation loop, added parameters for cell size
Jan 23, 2020
22f3acf
Add getop parameter parsing and help text output
Feb 7, 2020
5252078
Add output format parameter
Feb 7, 2020
1213f9e
Fix format
Feb 7, 2020
18d809e
Fix format
Feb 9, 2020
b239de6
Added support for HQ cam
Jun 24, 2021
8746d6a
Revert "Added support for HQ cam"
Jun 25, 2021
83d9275
Added support for HQ cam (with proper indentation)
Jun 25, 2021
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53 changes: 37 additions & 16 deletions lens_shading_analyse.c
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Original file line number Diff line number Diff line change
Expand Up @@ -316,27 +316,48 @@ int main(int argc, char *argv[])
uint16_t *channel = out_buf[channel_ordering[bayer_order][i]];
int mid_value_avg = 0;
int count = 0;
//Average out the centre 64 pixels.
for (x=(single_channel_width>>1)-4; x<=(single_channel_width>>1)+4; x++)
{
for (y=(single_channel_height>>1)-4; y<=(single_channel_height>>1)+4; y++)
{
mid_value_avg += channel[x + y*single_channel_width];
count++;
}
}
uint16_t middle_val = (mid_value_avg / count) << 5;

uint16_t *line;
const char *channel_comments[4] = {
"R",
"Gr",
"Gb",
"B"
};
printf("Middle_val is %d\n", middle_val);
};

// Search highest value
float max_val = 0;
for (y=0; y<grid_height; y++)
{
int y_start = y*32;
int y_stop = y_start + 32 <= single_channel_height ? y_start + 32 : single_channel_height;
for (x=0; x<grid_width; x++)
{
int x_start = x*32;
int x_stop = x_start + 32 <= single_channel_width ? x_start + 32 : single_channel_width;

float block_val = 0;
uint16_t block_px = 0;

for(int y_px = y_start; y_px < y_stop; y_px++){
line = &channel[y_px*(single_channel_width)];
for(int x_px = x_start; x_px < x_stop; x_px++){
block_val += line[x_px];
block_px++;
}
}

block_val /= block_px;
if(block_val > max_val){
max_val = block_val;
}
}
}

max_val *= 32;
printf("Max_val is %.0f\n", max_val);
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Without a better description I can't say for certain what this is meant to be doing, but it's very ugly to run the same loop twice, once to find this max_val, and then again to compute the cell gains.

Allocate arrays for block_val and block_px (if it's really necessary) and compute once. Calculating the gains then becomes an iteration over those arrays instead of the whole image.

(Sorry, embedded programming always makes you think of more efficient code).

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Running the loop twice is of course not a high-performance solution. But it runs very fast on my Raspberry Pi 3B ;-)
Quick and dirty - I will fix it.

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Double calculation loop removed

fprintf(header, "//%s - Ch %d\n", channel_comments[i], channel_ordering[bayer_order][i]);
uint16_t *line;

// Calculate gain for each block
for (y=0; y<grid_height; y++)
{
int y_start = y*32;
Expand All @@ -356,11 +377,11 @@ int main(int argc, char *argv[])
block_px++;
}
}
int gain = (middle_val*block_px) / block_sum;
int gain = (max_val*block_px) / block_sum + 0.5;
if (gain > 255)
gain = 255; //Clip as uint8_t
else if (gain < 32)
gain = 32; //Clip at x1.0
gain = 32; //Clip at x1.0, should never happen
fprintf(header, "%d, ", gain );
fprintf(table, "%d %d %d %d\n", x * 32 + 16, y * 32 + 16, gain, i );
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This feels wrong.
The ISP interpolates the values at the centre of each grid cell, and interpolates between them. This is averaging over the entire cell, therefore is just a smoothing filter.

Enlarging the average to 5x5, or even up as far as 9x9 would be reasonable, but otherwise you're ignoring a large amount of the detail.

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I do not know which algorithm is used for the interpolation. I don't see a problem with this approach. From my point of view and for my applications the results are better.

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I've been thinking again. If you look at lens shading across the width (alternatively across the height), the brightness will be a function similar to figure a. The average value changes only slightly with the width of the viewed area. However, the brightness signal from the sensor is subject to noise (figure b). A wider area improves the average value. Only in case of peaks smaller areas are better (figure c). In this case, however, the peak would have to lie exactly in a calculation area.

lens_shading

Or am I misunderstanding something?

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Integrated a parameter for the cell size.

uint8_t gain_bin = gain;
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