Anybody have a prgram to write a .bmp file?

[Robert A. Macy]

I need a program that writes an r, g, b set of matrices to
create a .bmp file

Anybody got one?

for example: here are the simple matrices:
>> r=ones(256,1)*(0:255)/255;
>> g=.5*ones(256,256);
>> b=.5*ones(256,256);

I tried to do 
>> imshow(r,g,b);
but I only get a solid colored image with no shading
variations.  the colors only come up based upon the first
few pixels

I tried to do 
>> [X,map]=rgb2ind(r,g,b);
>> saveimage("Xtest.ppm",X,"ppm",map);
but that produced an incorrect .ppm file that irfanview
turned into a series of striped lines.  

Anybody got a function that writes 24 bit color .bmp files?

Help!

        - Robert -