topic Re: IMX6 HDMI output formating in i.MX Processors

IMX6 HDMI output formating

xavgo — Mon, 20 Mar 2017 08:59:26 GMT

Dear all,

I noticed that the color levels are modified by the iMX6 HDMI output.

Here is the test I passed:

I generated monochrom images (ie : [240;0;0] [15;0;0]), I play them either using gstremer image freeze either an image processing application (ImageJ). I have an analyser at the HDMI output that shows that the codes are not anymore the code of the image file played. I made the same test with a PC on windows and the image levels are right the image file levels.

I am wondering if there is a transformation in the iMX6 display module (it seems to be like a gamma transformation or a LUT).

Does anyone knows about this issue? It there is suche a signal trnasformation, is there a way to bypass it?

Thank you very much for your help.

Regards.

Xavier

Re: IMX6 HDMI output formating

joanxie — Thu, 23 Mar 2017 08:11:01 GMT

do you mean hdmi output format is changed, but you don't find any weired on the hdmi showing?

Re: IMX6 HDMI output formating

xavgo — Fri, 24 Mar 2017 08:56:35 GMT

Hi,

Thank you for your answer.

Actually we have a specific use of the iMX6 HDMI output. We use the 24-bit RGB output to address 4 sub-pixels of our display :

HDMI[7-0] -> send to sub pixel 1

HDMI[15-8] -> send to sub pixel 2

HDMI[19-16] -> send to sub pixel 3

HDMI[23-17] -> send to sub pixel 4

On my PC, I generate an homogeneous backplane image (.PNG file) with the following level : 00x00 – 0x00 - 0xF0 and I am expecting to have the following result on the display :

Sub-pix1 OFF

Sub-pix2 OFF,

Sub pix3 OFF

Sub-pix4 ON 100%

This is the result that I actually have connecting my display to a PC output.

When I play the homogeneous backplane image with the iMX6 I do not have the same result:

Sub-pix1 OFF

Sub-pix2 OFF,

Sub pix3 ON ~50%

Sub-pix4 ON ~50%

I also noticed that the level applied to Sub-pix3 and Sub-pix4 changes if I use YUY2 or RGB as output format.

My guess is that there is color transformation applied by iMX6 which is the reason why we displayed colors are not the expected one (as it is with my PC).

Do you have any clue on how to make the HDMI output be fully linear (no offset, no gain, no gamma, no LUT…)?

Thank you very much for your help.

Regards.