Drawing a straight line produces artefacts

I believe I defined giP in the render function, will this do?

static PF_Err
Render(
  PF_InData *in_data,
  PF_OutData *out_data,
  PF_ParamDef *params[],
  PF_LayerDef *output)
{
  PF_Err err = PF_Err_NONE;
  AEGP_SuiteHandler suites(in_data->pica_basicP);
  /* Put interesting code here. */
  GainInfo giP;
  AEFX_CLR_STRUCT(giP);
  A_long linesL = 0;
  linesL = output->extent_hint.bottom - output->extent_hint.top;

Then the iteration8Suite:

ERR(suites.Iterate8Suite1()->iterate(in_data,
  0, // progress base
  linesL, // progress final
  &params[LINE_INPUT]->u.ld, // src 
  NULL, // area - null for all pixels
  (void*)&giP, // refcon - your custom data pointer
  MySimpleGainFunc8, // pixel function pointer
  output));