Triangle rasterization DirectX & Win32 Gdi - c++

Question

I'm currently following Andre Lamothe's book Tricks of the Windows Game Programming Gurus and trying to write my own software renderer with Win32 GDI. However, I got stuck at the triangle rasterization part of the book where the author is using Bresenham's line algorithm. I managed to get the rasterization to work on DirectX, but not on Win32 GDI despite identical rendering code. Both renders a triangle but the DirectX one renders it correctly while the Win32 almost renders it correctly
Link to my code: https://pastebin.com/VwySmQXS
It could be worth mention that the DirectX version is written in c++, while my own is written in C. I wasn't able to get a good screenshot of the two triangles so here is a picture painted in paint of how they both look.
Left is DirectX (how I want it to look), right is Win32 GDI:
Am I missing something here? Both draw_triangle_2d are called with the same arguments.
static void draw_top_tri(struct render_buffer *buffer,
int x1, int y1,
int x2, int y2,
int x3, int y3,
float r, float g, float b, float a)
{
unsigned int color = vec4_to_uint(vec4_mul(Vec4(r, g, b, a), 255.0f));
// this function draws a triangle that has a flat top
float dx_right, // the dx/dy ratio of the right edge of line
dx_left, // the dx/dy ratio of the left edge of line
xs, xe, // the starting and ending points of the edges
height; // the height of the triangle
int temp_x, // used during sorting as temps
temp_y,
right, // used by clipping
left;
// destination address of next scanline
unsigned int *dest_addr = NULL;
// test order of x1 and x2
if(x2 < x1)
{
temp_x = x2;
x2 = x1;
x1 = temp_x;
} // end if swap
// compute delta's
height = (float)(y3 - y1);
dx_left = (x3 - x1) / height;
dx_right = (x3 - x2) / height;
// set starting points
xs = (float)x1;
xe = (float)x2 + (float)0.5;
// perform y clipping
if(y1 < MIN_CLIP_Y)
{
// compute new xs and ys
xs = xs + dx_left * (float)(-y1 + MIN_CLIP_Y);
xe = xe + dx_right * (float)(-y1 + MIN_CLIP_Y);
// reset y1
y1 = MIN_CLIP_Y;
} // end if top is off screen
if(y3 > MAX_CLIP_X)
y3 = MAX_CLIP_X;
// compute starting address in video memory
dest_addr = (unsigned int *)buffer->memory + y1 * buffer->width;
// test if x clipping is needed
if(x1 >= MIN_CLIP_X && x1 <= MAX_CLIP_X &&
x2 >= MIN_CLIP_X && x2 <= MAX_CLIP_X &&
x3 >= MIN_CLIP_X && x3 <= MAX_CLIP_X)
{
// draw the triangle
for(temp_y = y1; temp_y <= y3; temp_y++, dest_addr += buffer->width)
{
memset((unsigned int *)dest_addr + (unsigned int)xs,
color, (unsigned int)(xe - xs + 1) * 4);
// adjust starting point and ending point
xs += dx_left;
xe += dx_right;
} // end for
} // end if no x clipping needed
else
{
// clip x axis with slower version
// draw the triangle
for(temp_y = y1; temp_y <= y3; temp_y++, dest_addr += buffer->width)
{
// do x clip
left = (int)xs;
right = (int)xe;
// adjust starting point and ending point
xs += dx_left;
xe += dx_right;
// clip line
if(left < MIN_CLIP_X)
{
left = MIN_CLIP_X;
if(right < MIN_CLIP_X)
continue;
}
if(right > MAX_CLIP_X)
{
right = MAX_CLIP_X;
if(left > MAX_CLIP_X)
continue;
}
memset((unsigned int *)dest_addr + (unsigned int)left,
color, (unsigned int)(right - left + 1) * 4);
} // end for
} // end else x clipping needed
} // end Draw_Top_Tri
static void
draw_bottom_tri(struct render_buffer *buffer,
int x1, int y1,
int x2, int y2,
int x3, int y3,
float r, float g, float b, float a)
{
unsigned int color = vec4_to_uint(vec4_mul(Vec4(r, g, b, a), 255.0f));
// this function draws a triangle that has a flat bottom
float dx_right, // the dx/dy ratio of the right edge of line
dx_left, // the dx/dy ratio of the left edge of line
xs, xe, // the starting and ending points of the edges
height; // the height of the triangle
int temp_x, // used during sorting as temps
temp_y,
right, // used by clipping
left;
// destination address of next scanline
unsigned int *dest_addr;
// test order of x1 and x2
if(x3 < x2)
{
temp_x = x2;
x2 = x3;
x3 = temp_x;
} // end if swap
// compute delta's
height = (float)(y3 - y1);
dx_left = (x2 - x1) / height;
dx_right = (x3 - x1) / height;
// set starting points
xs = (float)x1;
xe = (float)x1; // +(float)0.5;
// perform y clipping
if(y1 < MIN_CLIP_Y)
{
// compute new xs and ys
xs = xs + dx_left * (float)(-y1 + MIN_CLIP_Y);
xe = xe + dx_right * (float)(-y1 + MIN_CLIP_Y);
// reset y1
y1 = MIN_CLIP_Y;
} // end if top is off screen
if(y3 > MAX_CLIP_X)
y3 = MAX_CLIP_X;
// compute starting address in video memory
dest_addr = (unsigned int *)buffer->memory + y1 * buffer->width;
// test if x clipping is needed
if(x1 >= MIN_CLIP_X && x1 <= MAX_CLIP_X &&
x2 >= MIN_CLIP_X && x2 <= MAX_CLIP_X &&
x3 >= MIN_CLIP_X && x3 <= MAX_CLIP_X)
{
// draw the triangle
for(temp_y = y1; temp_y <= y3; temp_y++, dest_addr += buffer->width)
{
memset((unsigned int *)dest_addr + (unsigned int)xs,
color, (unsigned int)(xe - xs + 1) * 4);
// adjust starting point and ending point
xs += dx_left;
xe += dx_right;
} // end for
} // end if no x clipping needed
else
{
// clip x axis with slower version
// draw the triangle
for(temp_y = y1; temp_y <= y3; temp_y++, dest_addr += buffer->width)
{
// do x clip
left = (int)xs;
right = (int)xe;
// adjust starting point and ending point
xs += dx_left;
xe += dx_right;
// clip line
if(left < MIN_CLIP_X)
{
left = MIN_CLIP_X;
if(right < MIN_CLIP_X)
continue;
}
if(right > MAX_CLIP_X)
{
right = MAX_CLIP_X;
if(left > MAX_CLIP_X)
continue;
}
memset((unsigned int *)dest_addr + (unsigned int)left,
color, (unsigned int)(right - left + 1) * 4);
} // end for
} // end else x clipping needed
} // end Draw_Bottom_Tri}
ENGINE_CORE_EXPORT void
draw_triangle_2d(struct render_buffer *buffer,
int x1, int y1,
int x2, int y2,
int x3, int y3,
float r, float g, float b, float a)
{
// this function draws a triangle on the destination buffer
// it decomposes all triangles into a pair of flat top, flat bottom
int temp_x, // used for sorting
temp_y,
new_x;
// test for h lines and v lines
if((x1 == x2 && x2 == x3) || (y1 == y2 && y2 == y3))
return;
// sort p1,p2,p3 in ascending y order
if(y2 < y1)
{
temp_x = x2;
temp_y = y2;
x2 = x1;
y2 = y1;
x1 = temp_x;
y1 = temp_y;
} // end if
// now we know that p1 and p2 are in order
if(y3 < y1)
{
temp_x = x3;
temp_y = y3;
x3 = x1;
y3 = y1;
x1 = temp_x;
y1 = temp_y;
} // end if
// finally test y3 against y2
if(y3 < y2)
{
temp_x = x3;
temp_y = y3;
x3 = x2;
y3 = y2;
x2 = temp_x;
y2 = temp_y;
} // end if
// do trivial rejection tests for clipping
if(y3 < MIN_CLIP_Y || y1 > MAX_CLIP_X ||
(x1 < MIN_CLIP_X && x2 < MIN_CLIP_X && x3 < MIN_CLIP_X) ||
(x1 > MAX_CLIP_X && x2 > MAX_CLIP_X && x3 > MAX_CLIP_X))
return;
// test if top of triangle is flat
if(y1 == y2)
{
draw_top_tri(buffer, x1, y1, x2, y2, x3, y3, r, g, b, a);
} // end if
else
if(y2 == y3)
{
draw_bottom_tri(buffer, x1, y1, x2, y2, x3, y3, r, g, b, a);
} // end if bottom is flat
else
{
// general triangle that's needs to be broken up along long edge
new_x = x1 + (int)(0.5 + (float)(y2 - y1) * (float)(x3 - x1) / (float)(y3 - y1));
// draw each sub-triangle
draw_bottom_tri(buffer, x1, y1, new_x, y2, x2, y2, r, g, b, a);
draw_top_tri(buffer, x2, y2, new_x, y2, x3, y3, r, g, b, a);
} // end else
} // end Draw_Triangle_2D