Have you done step 1 yet? It look like it.
Then threshold the image with a fixed threshold that is just above the gray level of the chassis.
binaryImage = grayImage > threshold;
See the interactive thresholding utility/tool in my my File Exchange Also run the Image Segmentation Tutorial there.
Next you need to scan the blocks in your image. Assuming you have the rows and columns of the red lines, do something like this:
for xBlockIndex = 1 : length(columns)-1
col1 = columns(xBlockIndex);
col2 = columns(xBlockIndex+ 1) - 1;
for yBlockIndex = 1 : length(rows)-1
row1 = rows(yBlockIndex);
row2 = rows(yBlockIndex+1)-1;
% Extract sub image
subImage = grayImage(row1:row2, col1:col2);
% Extract mask sub image
subImageMask = binaryImage(row1:row2, col1:col2);
% Compute mean for pixel above the threshold.
meanGrayLevels(yBlockIndex, xBlockIndex) = mean(subImage(subImageMask));
end
end
