Color axis scaling
v = caxis
caxis controls the mapping of data values to the colormap. It affects any surfaces, patches, and images with indexed CData and CDataMapping set to scaled. It does not affect surfaces, patches, or images with true color CData or with CDataMapping set to direct.
caxis([cmin cmax]) sets the color limits to specified minimum and maximum values. Data values less than cmin or greater than cmax map to cmin and cmax, respectively. Values between cmin and cmax linearly map to the current colormap.
caxis auto computes the color limits automatically using the minimum and maximum data values. This is the default behavior. Color values set to Inf map to the maximum color, and values set to -Inf map to the minimum color. Faces or edges with color values set to NaN are not drawn.
caxis manual and caxis(caxis) freeze the color axis scaling at the current limits. This enables subsequent plots to use the same limits when hold is on.
Create (X,Y,Z) data for a sphere and view the data as a surface.
[X,Y,Z] = sphere; C = Z; surf(X,Y,Z,C)
Values of C have the range [-1 1]. Values of C near -1 are assigned the lowest values in the colormap; values of C near 1 are assigned the highest values in the colormap.
To map the top half of the surface to the highest value in the color table, use
To use only the bottom half of the color table, enter
which maps the lowest CData values to the bottom of the colormap, and the highest values to the middle of the colormap (by specifying a cmax whose value is equal to cmin plus twice the range of the CData).
resets axis scaling back to autoranging and you see all the colors in the surface. In this case, entering
Adjusting the color axis can be useful when using images with scaled color data. For example, load the image data and colormap for Cape Cod, Massachusetts. This command loads the image's data X and the image's colormap map into the workspace.
load cape % Display the image with CDataMapping set to scaled and % install the image's colormap. image(X,'CDataMapping','scaled') colormap(map) % This adjusts the color limits to span the range % of the image data, which is 1 to 192: caxis ans = 1 192
The blue color of the ocean is the first color in the colormap and is mapped to the lowest data value (1). You can effectively move sea level by changing the lower color limit value. For example, The following code compares four settings:
figure load cape colormap(map) subplot(2,2,1) image(X,'CDataMapping','scaled') axis image title('Caxis = [1 192]') subplot(2,2,2) image(X,'CDataMapping','scaled') axis image title('Caxis = [3 192]') caxis([3 192]) subplot(2,2,3) image(X,'CDataMapping','scaled') axis image title('Caxis = [5 192]') caxis([5 192]) subplot(2,2,4) image(X,'CDataMapping','scaled') axis image title('Caxis = [6 192]') caxis([6 192])
Surface, patch, and image graphics objects having indexed CData and CDataMapping set to scaled map CData values to colors in the figure colormap each time they render. CData values equal to or less than cmin map to the first color value in the colormap, and CData values equal to or greater than cmax map to the last color value in the colormap. The following linear transformation is performed on the intermediate values (referred to as C below) to map them to an entry in the colormap (whose length is m, and whose row index is referred to as index below).
index = fix((C-cmin)/(cmax-cmin)*m)+1; %Clamp values outside the range [1 m] index(index<1) = 1; index(index>m) = m;