Strange horizontal ghost effect mostly on light images

[nico23]

I'm experiencing a strange horizontal ghost/stripe effect mostly on light images (it's very visibile in text)

What could it be?

I'm using the STM32H7S78 microcontroller

[GaetanGodart]

Hello @nico23 ,

It looks like the issue doesn't only happen on text but also on the left of the flags.

Are you using a custom board or are you using the STM32H7S78-DK?

Regards,

[nico23]

Yes, it seems the issue happens on white/very bright pixels mostly

Yes, it's a custom board. These are the configurations for the display

static void MX_LTDC_Init(void)
{

  /* USER CODE BEGIN LTDC_Init 0 */

  /* USER CODE END LTDC_Init 0 */

  LTDC_LayerCfgTypeDef pLayerCfg = {0};

  /* USER CODE BEGIN LTDC_Init 1 */

  /* USER CODE END LTDC_Init 1 */
  hltdc.Instance = LTDC;
  hltdc.Init.HSPolarity = LTDC_HSPOLARITY_AH;
  hltdc.Init.VSPolarity = LTDC_VSPOLARITY_AH;
  hltdc.Init.DEPolarity = LTDC_DEPOLARITY_AH;
  hltdc.Init.PCPolarity = LTDC_PCPOLARITY_IIPC;
  hltdc.Init.HorizontalSync = 3;
  hltdc.Init.VerticalSync = 3;
  hltdc.Init.AccumulatedHBP = 11;
  hltdc.Init.AccumulatedVBP = 11;
  hltdc.Init.AccumulatedActiveW = 811;
  hltdc.Init.AccumulatedActiveH = 491;
  hltdc.Init.TotalWidth = 819;
  hltdc.Init.TotalHeigh = 499;
  hltdc.Init.Backcolor.Blue = 0;
  hltdc.Init.Backcolor.Green = 0;
  hltdc.Init.Backcolor.Red = 0;
  if (HAL_LTDC_Init(&hltdc) != HAL_OK)
  {
    Error_Handler();
  }
  pLayerCfg.WindowX0 = 0;
  pLayerCfg.WindowX1 = 800;
  pLayerCfg.WindowY0 = 0;
  pLayerCfg.WindowY1 = 480;
  pLayerCfg.PixelFormat = LTDC_PIXEL_FORMAT_RGB565;
  pLayerCfg.Alpha = 255;
  pLayerCfg.Alpha0 = 0;
  pLayerCfg.BlendingFactor1 = LTDC_BLENDING_FACTOR1_PAxCA;
  pLayerCfg.BlendingFactor2 = LTDC_BLENDING_FACTOR2_PAxCA;
  pLayerCfg.FBStartAdress = 0;
  pLayerCfg.ImageWidth = 800;
  pLayerCfg.ImageHeight = 480;
  pLayerCfg.Backcolor.Blue = 0;
  pLayerCfg.Backcolor.Green = 0;
  pLayerCfg.Backcolor.Red = 0;
  if (HAL_LTDC_ConfigLayer(&hltdc, &pLayerCfg, 0) != HAL_OK)
  {
    Error_Handler();
  }
  
  
  /* USER CODE BEGIN LTDC_Init 2 */

  /* USER CODE END LTDC_Init 2 */

}

I'm using I-Cache with SCB_EnableICache and

static void MX_DMA2D_Init(void)
{

  /* USER CODE BEGIN DMA2D_Init 0 */
  hdma2d.Instance = DMA2D;
  hdma2d.Init.Mode = DMA2D_R2M;
  hdma2d.Init.ColorMode = DMA2D_OUTPUT_RGB888;
  hdma2d.Init.OutputOffset = 0;
  /* USER CODE END DMA2D_Init 0 */
  
  if (HAL_DMA2D_Init(&hdma2d) != HAL_OK)
  {
    Error_Handler();
  }
  /* USER CODE BEGIN DMA2D_Init 2 */

  /* USER CODE END DMA2D_Init 2 */

}

and GPU2D

static void MX_GPU2D_Init(void)
{

  /* USER CODE BEGIN GPU2D_Init 0 */

  /* USER CODE END GPU2D_Init 0 */

  /* USER CODE BEGIN GPU2D_Init 1 */

  /* USER CODE END GPU2D_Init 1 */
  hgpu2d.Instance = GPU2D;
  if (HAL_GPU2D_Init(&hgpu2d) != HAL_OK)
  {
    Error_Handler();
  }
  /* USER CODE BEGIN GPU2D_Init 2 */

  /* USER CODE END GPU2D_Init 2 */

}

is there a specific thing I should look into to see what's wrong with the rendering?

[Ozone]

> (it's very visibile in text)

Which is basically proof that this is EMI / bandwidth related - the sections with the greatest signal contrast shows the greatest interference.

Display cable too long, or of insufficient quality ?

[nico23]

Hi @Ozone 

I've done other tries.

It turnes out the issue appears when I use a full background image. When there's no background image, no issue is detected. Please see below

[GGODA]

Hello @nico23 ,

Did you add an image or a box?

Can you try to add a box as a background? Try different colors.

What part of the display do you invalidate?

Regards,

[nico23]

Hi @GGODA 

the background is an image.

In the latest two images, I've just generated the code. No user code is added.

I'll try with a box as background

[Ozone]

I am not a hardware expert, but besides of external (cabling) issues, the display itself might be of low quality.

I very well remember "old" times, about a decade ago or more, when prolonged use of a terminal with static content did "burn in" this image, and a shadow of said image remained when the content changed.
This was very common with displays and operation terminals on industrial machinery running 24/7.

For a test, you could try vertical bars of different contrasts (colors) and width, to check the display visually.
A simple example would be black/white or blue/yellow.
I wouldn't be surprised if the visibilty of this bleed-over effect increases with change in magnitude of the individual pixel components (R/G/B).

[GGODA]

I did not want to assume a cheap display but that is also what it made me think of.

We could verify whether it is the display's fault by trying the same GUI on a different display if you have one.

Regards,

[Ozone]

Well, there is still the possibility of insufficient cabling, or you don't meet the requirements / specificiations of the display. Even an inappropriate power supply (for the display) can cause issues.

I would consult the responsible hardware guy, assuming you have one (and it is not you ...).