That line executes immediately after dd1 is created/set-up, so dd1value is whatever Value dd1 has initially. That is to say, that line executes before you have a chance to make a selection in dd1.
To be able to make selections and then use them for subsequent calculations, you need to make the program wait for the user. In a programmatic dialog like this, that typically means calling uiwait.
As for the question of having the selection made in dd1 change the visibility of dd3, you need to give dd1 a ValueChangedFcn, which executes when dd1's Value changes. In that function, you'll set the visibility of dd2 appropriately.
See the below code for those modifications. I also added a button the user clicks to signal that the selections are done, so the program should prepare the outputs and close the figure.
function [name,dd1_option,dd2_option,dd3_option] = Surface_Options()
fig = uifigure("Name", 'Single Beam Processing', 'Position', [30,75,1450,750]);
panel1 = uipanel(fig, "Position", [50,350,200,350]);
panel1.Title = "Program Setup";
panel1.TitlePosition = 'centertop';
panel1.FontSize = 14;
txtbox1 = uieditfield(panel1,"Text", "Position", [10,290,180,30], "Value", "Operator Name");
dd1 = uidropdown(panel1, 'Position', [10,250,180,30], 'ValueChangedFcn', @vcf_dd1);
dd1.Items = ["Choose Surface Type", "Depth", "Calculated Surface", "Fitted Plane", "Corrected Points", "None"];
dd1.ItemsData = [0 1 2 3 4 5];
dd1.Visible = 'on';
dd2 = uidropdown(panel1, 'Position', [10,210,180,30]);
dd2.Items = ["Choose Save Options", "Save as CSV", "Save as txt"];
dd2.ItemsData = [0 1 2];
dd1.Visible = 'on';
dd3 = uidropdown(panel1, 'Position', [10,170,180,30]);
dd3.Items = ["Choose Smoothing Type", "Lowess", "Cubic Interpolation"];
dd3.ItemsData = [0 1 2];
dd3.Visible = 'off';
uibutton(fig,'Position',[20 10 65 22],'Text','OK','ButtonPushedFcn',@bpf_btn);
% initialize outputs
name = txtbox1.Value;
dd1_option = dd1.Value;
dd2_option = dd2.Value;
dd3_option = dd3.Value;
% don't return until selections have been made and fig is closed
uiwait(fig);
function vcf_dd1(~,~)
switch dd1.Value
case 2
dd3.Visible = 'on';
otherwise
dd3.Visible = 'off';
end
end
function bpf_btn(~,~)
% update outputs
name = txtbox1.Value;
dd1_option = dd1.Value;
dd2_option = dd2.Value;
dd3_option = dd3.Value;
% close the figure
close(fig);
end
end
