Ok, problem solved. The code posted below is working, more or less. I was not properly locating objects on the underlying plane to benefit from the transparency.
Thanks for the help.
I'll go set up a dedicated test routine to get it working, so that if I can't figure it out I can post a short block of code.
It won't do any good to post anthing now, as its a large program, and short of showing the bitmap creation lines, most is not relevant.
I'll come back when I can better use your time...thanks.
Here is a test which runs but does not tint: It prints a red square on the target bitmap.
Note: this code loops endlessly...I use a breakpoint in the debugger to halt it to view the output.
ALLEGRO_BITMAP *airspeed_tape = NULL;
ALLEGRO_COLOR color_255 = al_map_rgb(255, 255, 255);
ALLEGRO_DISPLAY *display = NULL;
ALLEGRO_FONT *font = NULL;
int main(int argc, char **argv)
display = al_create_display(1280, 800);
airspeed_tape = al_create_bitmap(1280, 800);
float r =.5;
float g =.5;
float b = .5;
float a = .5;
ALLEGRO_COLOR transparent_color = al_map_rgba_f(a, a, a, a);
al_draw_tinted_bitmap(airspeed_tape, transparent_color, 0, 0, 0);