c - Image pixel scaling error - calculating SNR of CCD lens -


i trying calculate signal noise ratio of 2 loaded images taken same digital camera. idea calculate signal noise @ numerous light levels achieve graph shows signal noise ratio of ccd lens.

the problem facing @ moment that; when scale second image (image b) have same average pixel intensity first image (image a), value achieved not exact value first image. think problem result of rounding error image conversions.

the method use calculate snr is:

  1. convert images gray scale
  2. convert images 16 bit signed
  3. subtract image b a
  4. get average pixel intensity of subtraction
  5. subtract average value , b (to eliminate noise)
  6. convert 8 bit unsigned
  7. get average pixel values of both images
  8. get ratio of average values
  9. scale image b (16 bit unsigned) ratio average pixel value of image - problem area
  10. convert 8 bit , calculate average pixel intensity
  11. subtract images each other again eliminate last random noise
  12. calculate standard deviation of subtraction
  13. convert standard deviation variance

i new @ opencv , programming in general, better methods , on specific problem appreciated.

int _tmain(int argc, _tchar* argv[]) { float mean_a; float mean_b1; float var,mean_b; float grey_b_mul; float grey_sub2; float o, standard, r;  iplimage *img = cvloadimage("j:\\backup\\fotos\\img_0168.jpg", 3);//define images iplimage *img2 = cvloadimage("j:\\backup\\fotos\\img_0164.jpg", 3); iplimage *graya = cvcreateimage(cvgetsize(img),ipl_depth_8u, 1); iplimage *grayb = cvcreateimage(cvgetsize(img),ipl_depth_8u, 1); iplimage *gray1 = cvcreateimage(cvgetsize(img),ipl_depth_16u, 1); iplimage *gray1s = cvcreateimage(cvgetsize(img),ipl_depth_16s, 1); iplimage *gray2s = cvcreateimage(cvgetsize(img),ipl_depth_16s, 1); iplimage *gray2 = cvcreateimage(cvgetsize(img),ipl_depth_16u, 1); iplimage *sub1 = cvcreateimage(cvgetsize(img),ipl_depth_16s, 1); iplimage *imga = cvcreateimage(cvgetsize(img),ipl_depth_8u, 1); iplimage *imga1 = cvcreateimage(cvgetsize(img),ipl_depth_16s, 1); iplimage *imga1u = cvcreateimage(cvgetsize(img),ipl_depth_16u, 1); iplimage *imgb1u = cvcreateimage(cvgetsize(img),ipl_depth_16u, 1); iplimage *imgb1 = cvcreateimage(cvgetsize(img),ipl_depth_16s, 1); iplimage *imgb = cvcreateimage(cvgetsize(img),ipl_depth_8u, 1); iplimage *imgb_mul = cvcreateimage(cvgetsize(img),ipl_depth_16u, 1); iplimage *imgb_mull = cvcreateimage(cvgetsize(img),ipl_depth_16u, 1); iplimage *imgb_scaled = cvcreateimage(cvgetsize(img),ipl_depth_16s, 1); iplimage *imgbsc = cvcreateimage(cvgetsize(img),ipl_depth_8u, 1); iplimage *sub2 = cvcreateimage(cvgetsize(img),ipl_depth_16s, 1); iplimage *sub2u = cvcreateimage(cvgetsize(img),ipl_depth_16u, 1); iplimage *sub2_final = cvcreateimage(cvgetsize(img),ipl_depth_8u, 1);   if (img->nchannels ==3) {     cvcvtcolor(img, graya, cv_bgr2gray);    //convert grayscale cvcvtcolor(img2, grayb, cv_bgr2gray); } else {     cvcopyimage(img,graya); cvcopyimage(img2,grayb); }  cvscalar m = cvavg(graya, 0);   //calculate average pixel intensity double z = m.val[0]; cvscalar n = cvavg(grayb, 0);   //calculate average pixel intensity double y = n.val[0];  if (z > 128)            //convert 16 bit signed {     cvconvertscale(graya, gray1, 256.0, -32768);     cvconvertscale(gray1, gray1s, 1.0); } else     cvconvertscale(graya, gray1s, 256.0, -32768);  if (y> 128) {     cvconvertscale(grayb, gray2, 256.0, -32768);     cvconvertscale(gray2, gray2s, 1.0); } else     cvconvertscale(grayb, gray2s, 256.0, -32768);   cvnamedwindow("ccd noise",1); cvnamedwindow("image a",1); cvnamedwindow("image b scaled",1);    cvsub(gray1s,gray2s, sub1);     //subtract images cvsub(gray1s,sub1, imga1); cvsub(gray2s,sub1, imgb1);  cvconvertscale(imga1, imga1u, 1.0, +32768 ); cvconvertscale(imgb1, imgb1u, 1.0, +32768 ); cvconvertscale(imga1u, imga, 1.0/256); cvconvertscale(imgb1u, imgb, 1.0/256); cvscalar d = cvavg(imgb, 0);                                //calculate average pixel intensity mean_b = d.val[0];  cvscalar e = cvavg(imga, 0);                                //calculate average pixel intensity mean_a = e.val[0];  printf("image pixel intensity = %f\n", mean_a); printf("image b pixel intensity = %f\n", mean_b);  r = mean_a/mean_b;                                          // ratio between average pixel intensities of image , b printf("r = %f\n", r);   for( int = 0; < imgb1u->height; a++ )                   //scale image b achieve same average pixel intensity image {       for( int b = 0; b < imgb1u->width; b++ )     {            cvscalar pixel = cvget2d(imgb1u, a, b);          o = pixel.val[0]*r;         cvscalar p;         p.val[0] = o;         cvset2d(imgb_mul, a, b, p);     } } cvconvertscale(imgb_mul, imgbsc, 1.0/256);                  //convert 8 bit  cvscalar c = cvavg(imgbsc, 0);                              //calculate average pixel intensity mean_b1 = c.val[0];                                       printf("image b intensity after scaling = %f\n", mean_b1);  cvconvertscale(imgb_mul, imgb_scaled, 1.0, -32768);  cvsub(imga1,imgb_scaled, sub2);                             //final subtraction calculte standard deviation  cvconvertscale(sub2, sub2u, 1.0, +32768 ); cvconvertscale(sub2u, sub2_final, 1.0/256);  cvscalar t,std; cvavgsdv(sub2_final, &t, &std);                             //calculate std deviation standard = std.val[0]; var = (standard*standard)/2;                                //square std deviation , devide 2 variance  printf("variance camera noise = %f\n", var);  cvshowimage("image a",imga); cvshowimage("image b scaled",imgbsc); cvshowimage("ccd noise",sub2); cvwaitkey(0);                                   //to terminate images cvdestroywindow("ccd gain"); cvdestroywindow("image a"); cvdestroywindow("image b scaled"); cvreleaseimage(&img); cvreleaseimage(&img2); cvreleaseimage(&gray1); cvreleaseimage(&gray1s); cvreleaseimage(&gray2s); cvreleaseimage(&gray2); cvreleaseimage(&sub1); cvreleaseimage(&sub2); cvreleaseimage(&imga); cvreleaseimage(&imga1); cvreleaseimage(&imga1u); cvreleaseimage(&imgb); cvreleaseimage(&imgb1); cvreleaseimage(&imgb1u); cvreleaseimage(&imgb_mul); cvreleaseimage(&imgb_mull); cvreleaseimage(&imgbsc); cvreleaseimage(&imgb_scaled); cvreleaseimage(&sub2u); cvreleaseimage(&sub2_final);  return 0; }

the results of program are:

image pixel intensity = 138.292328

image b pixel intensity = 253.836456

ratio = 0.544809

image b intensity after scaling = 138.351196

variance of ccd noise = 8.016509

i used different images presentation here put emphasis on supposed happen. in testing images closer (ratio around 0.992).


Comments

Post a Comment

Popular posts from this blog

Why does Ruby on Rails generate add a blank line to the end of a file? -

running rails generate controller foo home results in: class foocontroller < application controller def home end end (there's nothing on line in actual file, blank line) is there purpose blank line? it's common use newline indicate end of line, last line. many editors (like vi, use) add in newline silently. consensus text files (especially in unix world) should end in newline, , there have been problems historically if wasn't present. why should text files end newline? the tool used count lines in file "wc" counts newlines in file, without trailing newline show 3 instead of 4. it improves readability of templates used in generator. consider: https://github.com/rails/rails/blob/master/railties/lib/rails/generators/rails/controller/templates/controller.rb to remove trailing newline, template have last line of: end<% end -%> instead of: end <% end -%> that seems less readable me. the discussion below refers
Read more

keyboard - Smiles and long press feature in Android -

i've created android keyboard. how add features inserting smilies, , using long keypresses display symbols? to create long-press behavior allows select several options pop-up menu, follow guide (specifically pop-up menu section): http://developer.android.com/guide/topics/ui/menus.html smileys (also known emoticons) combination of keys, understood programs , parsed same way. have do, when clicks smiley face on keyboard, simulate pressing of key combination matching emoticon. example, when clicks smiley, should simulate key presses ':' , ')' :). here's list of emoticons: http://en.wikipedia.org/wiki/list_of_emoticons
Read more

node.js - Bad Request - node js ajax post -

i'm using express 3.0 , node v0.11.0. have button submits form , i'm using ajax post data json object node server. client code is: $('#contact').submit(function(e) { console.log('submit called'); var formdata = json.stringify($('form').serializeobject()); console.log(formdata); $.ajax({ url: "http://localhost:3000/savedata/", type: "post", datatype: 'json', data: {objectdata: formdata}, contenttype: "application/json", complete: function() { console.log('process complete'); }, success: function(data) { console.log('process sucess'); }, error: function() { console.log('process error'); }, }); return false; }); then on server: var express = require('express'); var app = express(); app.set('views', __dirname + '/views'); app.set('view engine', 'jade'); app.use(express.bodyparser());
Read more