How can I use RGB values in an if-else condition

I'm testing a few gray-scale images. What approach would come handy when using x and y values on a pixel if my if statements were to rely of RGB Values?

The RGB values are equal to each other and the range is 0 to 1.

layout RGB [1 1 1];

Ex. if RGB>.25 use (x+1,y+1)

if RGB<.25<.5 use (x+1,y-3)

if RGB>.5 use (x+1,y)

What should I do if I want to use RGB values in my if-else statements?

[Geoff Hayes]

AJ516 - so is RGB the pixel value and you want to determine which neighbouring pixel value to "use" for "something"?

[AJ516]

I want to use the RGB of the pixel is the brightness of the pixel to create a sloped line. The value will determine the direction of y (either up, down or remain still). The if statements will test the values in 3 situations to tell where x2,y2 will be located.

Answer 1

[Walter Roberson]

Y = RGB(:,:,1);
xoff = zeros(size(Y));
yoff = zeros(size(Y));

xoff(:) = 1;           %you used the same for all combinations
yoff(:) = 0;           %for all values not overwritten below
yoff(Y < 0.5) = -3;    %order is important in these next statements
yoff(Y < 0.25) = 1;

[R, C] = ndgrid(1:size(Y,1), 1:size(Y,2));
idx = sub2ind(size(Y), R + yoff, C + xoff);    %note that x corresponds to columns not row
retrieved_values = Y(idx);