HDR blending not working with 16 bit images

I'm using the camresponse function and the makehdr function to blend few images and create a radiance map. I have no problem using 8 bits images, everything works fine. But if I use the same images but encoded with 16 bits, I get really bad results, as you can see in the following images. Using that crf with mkhdr results in a really bad quality hdr image.

The images are converted to 8 bit and 16 bit png from the original RAW file.

Any help? Is this function supposed to work with images with more than 8 bits? I guess so looking at camresponse.m but still I have these problems.

[Vinai Datta Thatiparthi]

Hey!

Both these commands that you mention should support 16 bit images as well.

Can you share the images and the code that you are working on?

[gbos]

clc; clear all;

%% Parameters
defaultT = [0.625,1/5,1/3,1.3,3];

%% Collection of photos
setDir = fullfile('img','*.png');
imds = imageDatastore(setDir);

figure(1);
montage(imds);

%Check if there is the exposure info
meta = imfinfo(imds.Files{1});
try
    temp = meta.DigitalCamera;
    exif = true;
catch
    exif = false;
end
        

%% Response Function
if(not(exif))
    crf = camresponse(imds,'ExposureTimes',defaultT);
else
    crf = camresponse(imds);
end
range = 0:length(crf)-1;
%Response function plots for every channel
figure(2); hold on;
if(size(crf,2) == 3)
    %RGB Image
    plot(crf(:,1),range,'--r','LineWidth',2);
    plot(crf(:,2),range,'-.g','LineWidth',2);
    plot(crf(:,3),range,':b','LineWidth',2);
    legend('R-component','G-component','B-component','Location','southeast');
else
    %Grayscale Image
    plot(crf(:,1),range,'-k','LineWidth',2);
end
xlabel('Log-Exposure');
ylabel('Pixel Value');
title('Camera Response Function');
grid on;
axis('tight');
hold off;

[gbos]

images can be downloaded here. They're different from the ones used to recover the crf I showed you on the first post, but you'll see the same problem shows up.

You can later try to convert the images in 8 bit PNGs and you'll see that the algorithm will work properly.

Answer 1

[Vinai Datta Thatiparthi]

Hey!

I believe that this is expected behavior of the commands makehdr and camresponse. 8 bit images generally produce smoother camresponse curves since the intensity range is narrower as compared to 16 bit images.

As a workaround to obtaining a more accurate looking curve, considering using a higher sampling interval to narrow down the “crf” result.

% Increasing sampling interval
crfNew = crf(1:200:65536,:);
range = 0:length(crfNew)-1;

% Response function plots for every channel
figure(2); hold on;
if(size(crf,2) == 3)
    % RGB Image
    plot(crfNew(:,1),range,'r','LineWidth',2);
    plot(crfNew(:,2),range,'g','LineWidth',2);
    plot(crfNew(:,3),range,'b','LineWidth',2);
    legend('R-component','G-component','B-component','Location','southeast');
else
    % Grayscale Image
    plot(crf(:,1),range,'-k','LineWidth',2);
end

Hope this helps!

[gbos]

Sadly it doesnt work. Weird things happen when I use 16 bit images. This is another example, with an RGB image. I tried using an higher sampling as you suggested.

8 bit PNG

16 bit PNG