Question

I am working on a project with a TFT touch screen. With this screen there is an included library. But after some reading, I still don’t get something. In the library there are some defines regarding colors:

/* some RGB color definitions                                                 */
#define Black           0x0000      /*   0,   0,   0 */
#define Navy            0x000F      /*   0,   0, 128 */
#define DarkGreen       0x03E0      /*   0, 128,   0 */
#define DarkCyan        0x03EF      /*   0, 128, 128 */
#define Maroon          0x7800      /* 128,   0,   0 */
#define Purple          0x780F      /* 128,   0, 128 */
#define Olive           0x7BE0      /* 128, 128,   0 */
#define LightGrey       0xC618      /* 192, 192, 192 */
#define DarkGrey        0x7BEF      /* 128, 128, 128 */
#define Blue            0x001F      /*   0,   0, 255 */
#define Green           0x07E0      /*   0, 255,   0 */
#define Cyan            0x07FF      /*   0, 255, 255 */
#define Red             0xF800      /* 255,   0,   0 */
#define Magenta         0xF81F      /* 255,   0, 255 */
#define Yellow          0xFFE0      /* 255, 255,   0 */
#define White           0xFFFF      /* 255, 255, 255 */
#define Orange          0xFD20      /* 255, 165,   0 */
#define GreenYellow     0xAFE5      /* 173, 255,  47 */
#define Pink                        0xF81F

Those are 16-bit colors. But how do they go from: 0, 128, 128(dark cyan) to 0x03EF. I mean, how do you convert a 16-bit color to a uint16? This doesn’t need to have an answer in code, because I just want to add some colors in the library. A link to a online converter (which I could not find) would be okay as well 🙂

Thanks

Answer 1

From these one can easily find out the formula:

#define Red             0xF800      /* 255,   0,   0 */  
#define Magenta         0xF81F      /* 255,   0, 255 */
#define Yellow          0xFFE0      /* 255, 255,   0 */

F800 has 5 MSB bits set and FFE0 has 5 LSB not set.
0xF81F has obviously both 5 LSB’s and 5 MSB’s set, which proves the format to be RGB565.

The formula to convert a value 173 to Red is not as straightforward as it may look — you can’t simply drop the 3 least significant bits, but have to linearly interpolate to make 255 to correspond to 31 (or green 255 to correspond to 63).

NewValue = (31 * old_value) / 255;

(And this is still just a truncating division — proper rounding could be needed)

With proper rounding and scaling:

Uint16_value = (((31*(red+4))/255)<<11) | 
               (((63*(green+2))/255)<<5) | 
               ((31*(blue+4))/255);

EDIT Added parenthesis to as helpfully suggested by JasonD.