IMFTransfomer::ProcessInput() and MF_E_TRANSFORM_NEED_MORE_INPUT - c++

Question

I have code that decodes AAC-encoded audio using IMFTransform. It works well for various test inputs. But I observed that in some cases IMFTransform::ProcessOutput() returns MF_E_TRANSFORM_NEED_MORE_INPUT when according to my reading of MS documentation it should return a valid data sample.
Basically the code has the following structure:
IMFTransform* transformer;
MFT_OUTPUT_DATA_BUFFER output_data_buffer;
...
bool try_to_get_output = false;
for (;;) {
if (try_to_get_output) {
// Try to get the outpu sample.
try_to_get_output = false;
output_data_buffer.dwStatus = 0;
...
hr = transformer->ProcessOutput(...&output_data_buffer);
if (success) {
// process sample
if (output_data_buffer.dwStatus & MFT_OUTPUT_DATA_BUFFER_INCOMPLETE) {
// We have more data
try_to_get_output = true;
}
} else if (hr == MF_E_TRANSFORM_NEED_MORE_INPUT) {
Log("Unnecessary ProcessOutput()");
} else {
// Process other errors
}
continue;
}
// Send more encoded AAC data to MFT.
hr->ProcessInput();
}
What happens is that ProcessOutput() sets MFT_OUTPUT_DATA_BUFFER_INCOMPLETE in MFT_OUTPUT_DATA_BUFFER.dwStatus but then the following ProcessOutput() always returns MF_E_TRANSFORM_NEED_MORE_INPUT contradicting the documentation.
Again, so far it seems harmless and things works. But then what exactly does AAC decoder want to tell the caller via setting MFT_OUTPUT_DATA_BUFFER_INCOMPLETE?

Answer 1

This might be a small glitch in the decoder implementation. Quite possible that if you happen to drain the MFT it would spit out some data, so the incompletion flag migth indicate, a bit confusingly, some data even though not immediately accessible.
However overall the idea is to do ProcessOutput sucking the output data for as long as possible until yщu get MF_E_TRANSFORM_NEED_MORE_INPUT, and then proceed with feeding new input (or draining). That is, I would say MF_E_TRANSFORM_NEED_MORE_INPUT is much more important compared to MFT_OUTPUT_DATA_BUFFER_INCOMPLETE. After all this is what Microsoft's own code over MFTs does.
Also keep in mind that AAC decoder is an "old", "first generation" MFT and so over years its update could be such that it diverted a bit from the current docs.