TDA4VM: LDC parameters

PL Tanguay

Part Number: TDA4VM

Regarding VPAC LDC, we would like to optimize the similarity between an LDC undistorted output frame VS a CPU openCV undistorted output frame.

We believe we have built a valid mesh table, with the correct data format (S16Q3). Our results are very good using subsampling factor=0 (tested with region out block size of 8x8, 16x16 and 32x32). By good results I mean the LDC output undistorted frame is very similar to frame that was undistorted using CPU OpenCV.

Question 1: We would like to increase the subsampling factor so we can have better performance on the VPAC LDC, but also by keeping and acceptable similarity. Would you say there is a specific subsampling factor we should reach to be acceptable in a real application usage? I mean by that, is there a gold standard factor for real time usage, or would it be case by case?

Question 2: We noticed there are green/black artifacts along the bottom and right side of the output frame starting subsampling factor=2. We tried different region parameters out block sizes, the artifacts got smaller, but is there something else we should be looking into? Is there any other critical parameters we should consider?

Question 3: What would be an acceptable performance (in ms) to undistort a 4K frame using LDC?

Thank you!

over 2 years ago

0 Gang Hua over 2 years ago

TI__Guru 62480 points

Hi,

In case you are not aware of it, we have a FAQ page for LDC: https://e2e.ti.com/support/processors-group/processors/f/processors-forum/1058565/faq-tda4vm-how-to-create-a-ldc-mesh-lut-for-fisheye-distortion-correction-on-tda4?keyMatch=LDC%20FAQ

Q1.

I typically recommend using m=3 (8x8) or m=4 (16x16) for sum-sampling (no down-sampling is only for debugging).
There should be no visual difference with either.

Q2.

This is typically due to issues in the mesh LUT.
When you use the tuning tool to try your input image and LUT, do you see any error/warning messages?

Q3.

LDC runs at 720MHz or 640MHz (depending on clock settings) with 1 pixel per clock steady state performance (in bi-linear mode which almost everyone uses).
You may calculate the theoretical LDC execution time given your output image size.
If your particular mesh mapping requires large DDR bandwidth for LDC reading its input images, then LDC may be slowed down.
Of course, with m=0 (no mesh down-sampling), LDC will run very slowly.

0 PL Tanguay over 2 years ago in reply to Gang Hua

Prodigy 110 points

Thank you,

It appears the output frame sometimes doesn't have the same result. I mean, I just tried a couple of times with the same parameters and I noticed sometimes there are much less amount of greens block.

Edit: I notice I don't have green block when I reboot the target and run with m=2 (64x64). They usually show up after multiple runs

Edit 2: Sorry I meant m=2 with region out block size 64x64

0 Gang Hua over 2 years ago in reply to PL Tanguay

TI__Guru 62480 points

Hi,

Green blocks are typically from unexpected 0 values in the UV channel.
BTW, m=2 is for 4x4 mesh LUT down-sampling.

One possibility is that there is some continuity issue in the mesh LUT.
With mesh interpolation, it may result in very large LDC input block size so that LDC runs out of internal memory.

If you follow the LDC FAQ, you should be able to test the mesh LUT and get the error/warning messages in DCC tuning tool.

0 PL Tanguay over 2 years ago in reply to Gang Hua

Prodigy 110 points

Sorry I meant m=2 with region out block size 64x64

+1 Gang Hua over 2 years ago in reply to PL Tanguay

TI__Guru 62480 points

No problem.

A large output block size may result in LDC internal memory overflow.

The tuning tool will give you the best output block size and pixpad value given your mesh LUT.

0 PL Tanguay over 2 years ago in reply to Gang Hua

Prodigy 110 points

Thank you, currently working on the Tuning Tool!

0 PL Tanguay over 2 years ago

Prodigy 110 points

After using the DCC Tuning tool, it looks like I'm having a warning at Bit0:

I'm not sure of the meaning of it though..

FYI I have these result as my top 5 set of parameters:

+1 Gang Hua over 2 years ago in reply to PL Tanguay

TI__Guru 62480 points

Thanks for the update!
This looks OK.

The warning means going out of input image boundary, which should not cause any artifacts.
You should not see any issue in the output image in tuning tool with the recommended settings (e.g., 128x48).

You may export the settings to an xml file from the tuning tool and use it in PSDK/EVM.
Do you see any artifacts on EVM?

0 PL Tanguay over 2 years ago in reply to Gang Hua

Prodigy 110 points

No I don't see any artifacts! Thank you

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TDA4VM: LDC parameters