+/**
+ * Compute a0 for a constant-valued coefficient (GL_FLAT shading).
+ * The value value comes from vertex[slot][i].
+ * The result will be put into setup->coef[slot].a0[i].
+ * \param slot which attribute slot
+ * \param i which component of the slot (0..3)
+ */
+static void const_pos_coeff( struct setup_context *setup,
+ uint vertSlot, unsigned i)
+{
+ setup->coef.dadx[0][i] = 0;
+ setup->coef.dady[0][i] = 0;
+
+ /* need provoking vertex info!
+ */
+ setup->coef.a0[0][i] = setup->vprovoke[vertSlot][i];
+}
+
+
+/**
+ * Compute a0 for a constant-valued coefficient (GL_FLAT shading).
+ * The value value comes from vertex[slot][i].
+ * The result will be put into setup->coef[slot].a0[i].
+ * \param slot which attribute slot
+ * \param i which component of the slot (0..3)
+ */
+static void const_coeff( struct setup_context *setup,
+ unsigned attrib,
+ uint vertSlot)
+{
+ unsigned i;
+ for (i = 0; i < NUM_CHANNELS; ++i) {
+ setup->coef.dadx[1 + attrib][i] = 0;
+ setup->coef.dady[1 + attrib][i] = 0;
+
+ /* need provoking vertex info!
+ */
+ setup->coef.a0[1 + attrib][i] = setup->vprovoke[vertSlot][i];
+ }
+}
+
+
+/**
+ * Compute a0, dadx and dady for a linearly interpolated coefficient,
+ * for a triangle.
+ */
+static void tri_linear_coeff( struct setup_context *setup,
+ unsigned attrib,
+ uint vertSlot)
+{
+ unsigned i;
+ for (i = 0; i < NUM_CHANNELS; ++i) {
+ float botda = setup->vmid[vertSlot][i] - setup->vmin[vertSlot][i];
+ float majda = setup->vmax[vertSlot][i] - setup->vmin[vertSlot][i];
+ float a = setup->ebot.dy * majda - botda * setup->emaj.dy;
+ float b = setup->emaj.dx * botda - majda * setup->ebot.dx;
+ float dadx = a * setup->oneoverarea;
+ float dady = b * setup->oneoverarea;
+
+ assert(i <= 3);
+
+ setup->coef.dadx[1 + attrib][i] = dadx;
+ setup->coef.dady[1 + attrib][i] = dady;
+
+ /* calculate a0 as the value which would be sampled for the
+ * fragment at (0,0), taking into account that we want to sample at
+ * pixel centers, in other words (0.5, 0.5).
+ *
+ * this is neat but unfortunately not a good way to do things for
+ * triangles with very large values of dadx or dady as it will
+ * result in the subtraction and re-addition from a0 of a very
+ * large number, which means we'll end up loosing a lot of the
+ * fractional bits and precision from a0. the way to fix this is
+ * to define a0 as the sample at a pixel center somewhere near vmin
+ * instead - i'll switch to this later.
+ */
+ setup->coef.a0[1 + attrib][i] = (setup->vmin[vertSlot][i] -
+ (dadx * (setup->vmin[0][0] - setup->pixel_offset) +
+ dady * (setup->vmin[0][1] - setup->pixel_offset)));
+
+ /*
+ debug_printf("attr[%d].%c: %f dx:%f dy:%f\n",
+ slot, "xyzw"[i],
+ setup->coef[slot].a0[i],
+ setup->coef[slot].dadx[i],
+ setup->coef[slot].dady[i]);
+ */
+ }
+}
+
+