Turn Down Opacity or Hide Layer

Question

Hi @ everybody,
1) I would like to turn on/off the layer (toggle hide) or turn down the opacity to 100%. Can you explain me how i can do that?
I did a littl research and found just one forum post about opacity:
PF_compositeMode
    compositeMode.opacity   = PF_MAX_CHAN8; // 0 - 255
would be nice if someone could give me a hint
2) I've played around with the "resize"-example by adding currentframenumber (in-data->current_time / in-data->time_step) to the border-size to give it a dynamic changement. I've recognized that it only works when i add keyframes to that effect. I think i did not totally understand how to handle with specific variables.
the change i've made:
out_data->width  = 2 * static_cast<A_long>(border_x) + params[0]->u.ld.width +  6* (in_data->current_time / in_data->time_step);
out_data->height = 2 * static_cast<A_long>(border_y) + params[0]->u.ld.height + 6* (in_data->current_time / in_data->time_step) ;
i thought, that as soon as i click play, the timeline is increasing, that the changes will be made.
by the way what is
u.ld.width

Answer 1

You can use the AEGP_SetLayerFlag like every other suite function. Have a look at the following code snippet.
AEGP_SuiteHandler suites(in_data->pica_basicP); //the suite handler
AEGP_LayerH your_layer = whatever your layer is
A_Boolean valueB = FALSE; //True if you want to turn it on
suites.LayerSuite6()->AEGP_SetLayerFlag(your_layer,AEGP_LayerFlag_VIDEO_ACTIVE,valueB);
Looking at your expression, there is no way you can achieve that using layer flags, because they are not keyframed. So you have to use the opacity of the layer and streams. To understand streams, there are many example projects in the SDK like Streamie etc etc.,
But, I highly suggest you to do this using AE Scripting instead of going in the plugin route. Trust me, it saves you a lot of time and debugging.
If you are that particular of creating a plugin, another way is to calculate your time interval based on the time given by the In_data and then fill the image buffer with NULL (Basically, transparent pixels) so that it gives you a zero opacity effect. That'll save you from using streams.
BTW, I didnt see your expression in great detail, but isn't that similar to the "Strobe Light" effect that comes with AE by default. I am not sure, but you might wanna have a look at it.