Dear Alstan, Dear all, >Clotilde, there is one notable place where my workflow for calibration factors >is a little different than yours. I calibrate each image based on a specific >luminance >measurement as per Inanici and Van Den Wymelenberg's methods. >Generally speaking, this brings my results reasonably close to the measured >illuminances in >most cases where overflow is not present. I keep the >measurement point near the image center (where vignetting~0) and perform >vignetting correction after >calibrating the luminance at the near-center >point.
Well, I also take several luminance measurements each time I capture a LDR series. I was planning to use these measurements as a means of verification (rather than a means of calibration) to be sure I got the right luminance values with my HDR. But anyway, I imagine that using an in-situ luminance measurement instead of a previously derived calibration factor to calibrate an HDR images does not have a great impact on the results. Both calibration factors (the one derived beforehand and the one derived for each HDR image) should be similar, shouldn’t they? I still have tried to calibrate my HDR image with an in-situ luminance measure: With the Canon EOS 5D Mark II and a Sigma fisheye EX DG 8mm (aperture set on f/11, WB on daylight and all other settings on neutral), I took a JPG series of 17 images (shutter speed going from 1/8000 to 5s). Then I picked a sequence of 12 images out of these 17 jpg images (until there is no pixel <20 in the lightest exposure and no pixel >200 in the darkest exposure). To derive the HDR image, I used hdrgen –a –e –f –g and a response curve I derived previously. I applied the vignetting calibration and the reprojection with the pcomb command in Radiance. I cropped the image to a square (1000x1000) and modified the header (view and exposure). Using Photosphere, I applied a calibration factor on the central grey card (the HDR gave 77.1cd/m² and I measured 111.9cd/m²). The calibration factor used is 1.45, which is not so far from the 1.48 I was using before. For the other luminance values I measured in the scene, I got these values with the calibrated HDR: 2660 cd/m² (HDR) VS 2817 cd/m² (real) 42.4 cd/m² (HDR) VS 41.04 cd/m² (real) 1740 cd/m² (HDR) VS 1516 cd/m² (real) And for the illuminance value, I measured 550lux and I got 605lux with the calibrated HDR. These are still big differences in my opinion, and not always on the same side. What do you think? >Secondarily, I'm curious about your vignetting correction process before >cropping if you wouldn't mind sharing some more details. I imagine it is >difficult to >correct when there is no valid Radiance view associated with the >image yet. Well, I derived my vignetting curves exactly the same way that Cauwerts used (we are from the same lab). Then, from these curves, I created .cal files (one for each aperture I use). For instance, I have this .cal file for aperture f/11 sq(x)=x*x; r=sqrt(sq(x-2825)+sq(y-1879))/1775; (where I define the radius of the circular image where to apply the vignetting using the pixel coordinates of the centrum) sf=1/(1+(K_1*r^1)+(K_2*r^2)+(K_3*r^3)+(K_4*r^4)+(K_5*r^5)+(K_6*r^6)+(K_7*r^7)+(K_8*r^8)); (with K_x the factors I defined) ro=sf*ri(1); go=sf*gi(1); bo=sf*bi(1); Then, I simply use the command: pcomb -f calFile input.hdr > output.hdr Also, I am not sure how the x option works in hdrgen but it seems it is not 100% reliable. I thus decided to manually pick the sequence of input JPG images, removing too light or too dark images myself. Best, Clotilde _______________________________________________ HDRI mailing list [email protected] http://www.radiance-online.org/mailman/listinfo/hdri
