Difference between pcolor and contourf

I'm trying to understand what the difference is between pcolor and contourf. If I have a 2D data, stored in variable Z, that I would like to plot, is there a difference between:

pcolor(X,Y,Z)
shading flat

AND

contourf(X,Y,Z)
shading flat

?

Answer 1

[Image Analyst]

contourf uses isolines and has uniform colors in between the iso lines. pcolor doesn't do that at all. pcolor, in my opinion is deceptive because it throws away the last row and column and shows you an array (rows-1) by (columns-1). I see virtually no advantage of pcolor over image() or imshow().

[David C]

"contourf..has uniform colors in between the iso lines" - you mean contourf interpolates between the isolines right? So it will add a gradient even if my data does not have the gradient?

[Image Analyst]

No. Interpolates means the value changes. What would you get if you did that? You'd get the original image, which you already have. Look at the demos and see that the entire zone between two isolines is the same color - it's uniformly blue or red or whatever. It's not interpolating or changing.

Answer 2

[Star Strider]

The pcolor function uses the values of the adjacent points to determine the colors of the enclosed cell. So the color is defined by the points enclosing the cell, not the value of the function within the cell. From the documentation for pcolor (where C is its (MxN) matrix argument):

• A pseudocolor plot is a rectangular array of cells with colors determined by C. MATLAB® creates a pseudocolor plot using each set of four adjacent points in C to define a surface rectangle (i.e., cell).

Give the same data to both pcolor, image and contourf to see the difference.

[David C]

Thanks - this is useful info. Too bad I can only accept one answer.

[Image Analyst]

To make up for that, I gave him 2 points by voting for it. You can also vote in addition to accepting (if you haven't already).

[Star Strider]

Thank you both!

Answer 3

[Shafiullah]

please, help me I am implementing contours on mammogram image, Although I am getting right segmentation but i want the contour to be filled inside with some color.

[c h] = contour(u ,[0 0],'linecolor','r'); is simple contour with red colored line, now i want same contour but filled inside with redcolor.

i have tried contourf(u ,[0 0],'faceColor','r'); but it causes my whole image darker and invisible.

Suggest me please.

[Image Analyst]

Do your segmentation to get a mask (binary image), then use patch() or fill().

Also see these:

http://blogs.mathworks.com/steve/2009/02/18/image-overlay-using-transparency/

http://www.mathworks.com/matlabcentral/answers/104889-how-to-overlay-a-foreground-image-over-a-background-image