AndreAhmed AndreAhmed - 23 days ago 7
C Question

Mapping intensity to color value in flat shading

I have a 16bit RGB565 and I have the result from dot product and its 0->1.

I would like to map the dot product value from light source to the normal vector in a mesh to 16bit color value. so I have a dot produc from 0>1 and I would like to modulate the color which is 16bit RGB565

uint16_t floatToRGB565(float input)
{
uint8_t x = input*255;
uint8_t r = (x&248u);//0xff-0x07, 5msb mask
uint8_t g = (x&252u);//0xff-0x03, 6msb mask
uint8_t b = (x&248u);//0xff-0x07, 6msb mask

return (r<<8)|(g<<3)|(b>>3);// assuming r is at msb
}



dp = Dot_Product_3D((vector_3d_ptr)&normal,
(vector_3d_ptr)&light_source);

// test if light ray is reflecting off surface


if (dp > 0)
{

float intens ;
uint16_t colorMod;
int r,g,b;
float Red, Green, Blue, result;
intensity = ambient_light + dp;

if (intensity > 1)
intensity = 1;
if (intensity < 0)
intensity = 0;
r = the_object->polys[curr_poly].color >> 11;
g = (the_object->polys[curr_poly].color >>5)&0x00FF;
b = the_object->polys[curr_poly].color & 0x0000FF;
Red = r*intensity;
Green = g*intensity;
Blue = b*intensity;
result = (int)(Red * 31) << 11 | (int)(Green * 63) << 5 | (int)(Blue * 31);
colorMod = floatToRGB565(result);
the_object->polys[curr_poly].shade = colorMod;
// printf("\nintensity of polygon %d is %f",curr_poly,intensity);

} // end if light is reflecting off surface
else{
int r,g,b;
float Red, Green, Blue, result;
float colorModulate;
uint16_t colorMod;
r = the_object->polys[curr_poly].color >> 11;
g = (the_object->polys[curr_poly].color >>5)&0x00FF;
b = the_object->polys[curr_poly].color & 0x0000FF;
Red = r*ambient_light;
Green = g*ambient_light;
Blue = b*ambient_light;
colorModulate = (int)(Red * 31) << 11 | (int)(Green * 63) << 5 | (int)(Blue * 31);
colorMod = floatToRGB565(colorModulate);
the_object->polys[curr_poly].shade = colorMod;
}
} // end if use flat shading

Answer
  1. dot product returns range <-1,+1>

    but your condition dp>0 handles it so that should not be the problem.

  2. float to RGB565 conversion

    You are missing some checks I would do it like this:

    uint16_t floatToRGB565(float input)
         {
         uint8_t x = float(input*63.0); // just to be sure it would not be rounded to int before multiplication
         if (x>63) x=63; if (x<0) x=0; // avoid overflow/underflow you never know with floats ...
         uint8_t r = x>>1;
         uint8_t g = x;
         uint8_t b = x>>1;
         return (r<<11)|(g<<5)|(b); // assuming r is at msb
         }
    
Comments