correct glsl affine texture mapping

Question

i\'m trying to code correct 2D affine texture mapping in GLSL.

Explanation:

...NONE of thi

Answer 1

I had similar question ( https://gamedev.stackexchange.com/questions/174857/mapping-a-texture-to-a-2d-quadrilateral/174871 ) , and at gamedev they suggested using imaginary Z coord, which I calculate using the following C code, which appears to be working in general case (not just trapezoids):

//usual euclidean distance
float distance(int ax, int ay, int bx, int by) {
  int x = ax-bx;
  int y = ay-by;
  return sqrtf((float)(x*x + y*y));
}

void gfx_quad(gfx_t *dst //destination texture, we are rendering into
             ,gfx_t *src //source texture
             ,int *quad  // quadrilateral vertices
             )
{
  int *v = quad; //quad vertices
  float z = 20.0;
  float top = distance(v[0],v[1],v[2],v[3]); //top
  float bot = distance(v[4],v[5],v[6],v[7]); //bottom
  float lft = distance(v[0],v[1],v[4],v[5]); //left
  float rgt = distance(v[2],v[3],v[6],v[7]); //right

  // By default all vertices lie on the screen plane
  float az = 1.0;
  float bz = 1.0;
  float cz = 1.0;
  float dz = 1.0;

  // Move Z from screen, if based on distance ratios.
  if (top

I'm doing it in software to scale and rotate 2d sprites, and for OpenGL 3d app you will need to do it in pixel/fragment shader, unless you will be able to map these imaginary az,bz,cz,dz into your actual 3d space and use the usual pipeline. DMGregory gave exact code for OpenGL shaders: https://gamedev.stackexchange.com/questions/148082/how-can-i-fix-zig-zagging-uv-mapping-artifacts-on-a-generated-mesh-that-tapers