Darker horizontal line/strip when using gradient for red color only

[Qrenz]

Hi, I'm using FT811, when i use gradient color via CMD_GRADIENT there're some horizontal darker line/strip appearing on the screen. This is only happening when i use gradient, and only for red color (blue and green no darker line/strip occuring).
Here's my LCD parameter:

Code Select Expand

// LCD display parameters
UINT16 lcdWidth = 800; // Active width of LCD display
UINT16 lcdHeight = 480; // Active height of LCD display
UINT16 lcdHcycle = 928; // tot length of line (hoffset + data + front porch)
UINT16 lcdHoffset = 88; // start of active/data line ((hsync1 - hsync0) + back porch)
UINT16 lcdHsync0 = 0; // start of hsync
UINT16 lcdHsync1 = 48; // end of hsync
UINT16 lcdVcycle = 525; // total number of line (voffset + data + front porch)
UINT16 lcdVoffset = 32; // start of active/data line ((vsync1 - vsync0) + back porch)
UINT16 lcdVsync0 = 0; // start of vsync
UINT16 lcdVsync1 = 4; // end of vsync
UINT8 lcdPclk = 2; // Pixel Clock = 60 MHz / 2 = 30 MHz (LCD spec min-typ-max 20-33-50MHz)
UINT8 lcdSwizzle = 0; // Define RGB output pins
UINT8 lcdPclkpol = 1; // PCLK polarity is on the falling edge

And I was observing, if I make the clock higher (PCLK = 1) the line become lesser, but if I reduce the clock (PCLK = 3, 4, 5) the dark strip become many.
And the dark strip only happen for horizontal gradient applied, it's smooth if i use vertical gradient.

What could be the cause of this? How can i eliminate this completely?
Is this due to FT811 not a true RGB only 18 bit?

[BRT Community]

Hello,

Can you provide an example of the gradient you are seeing these issues on?
CMD_GRADIENT will calculate a blend from one colour to another, the use of 18bit RGB may effect whats output on the panel, along with the algorithm used to calculate the blend. You may find bitmaps a better option to provide a background gradient from tow colours which closely match each other.


Best Regards,
BRT Community

[NSTtom]

Was this ever solved? I am having the same issue when drawing a vertical gradient from orange to red, there is always a thin black line towards the red end but not on any other colors across any of the other gradients I have done. Turns out this issue is across gradients and RECTS that use the RED color

[BRT Community]

Hello,

we'd like to offer the following explanations and suggestions:
1. Mismatch in RGB Bit Depth (OUTBITS Setting)
Some LCD panels, although labeled as supporting RGB888 (24-bit), may internally operate using RGB666 (18-bit). If the EVE IC's output bit setting does not match the actual bit depth supported by the display, it can lead to color distortion or visual artifacts such as noise lines.
Suggestion:
Please verify your LCD panel's actual color depth and ensure that the REG_OUTBITS register is configured appropriately during initialization to match the panel.

2. Avoid Using CMD_GRADIENT for Red-Sensitive Precision Areas
CMD_GRADIENT uses internal linear interpolation. In high-contrast or precision-sensitive areas—such as gradients involving red—it is prone to rounding errors that may cause visible banding or fine lines.
Suggestion:
Consider using a custom gradient image (bitmap) instead, and load it into RAM or flash to ensure consistent rendering and avoid interpolation artifacts.

3. Fine-Tune the Gradient End Color & Refresh the Cache
Using red values like 0xFF0000 (full red) can fall on edge-case transitions in RGB666 mode, leading to visual inconsistencies.
Suggestion:
Try slightly dimming the gradient end color—for example, use 0xF00000 instead of 0xFF0000—and observe whether the black line still appears.
Also, try calling CMD_CLEARCACHE() to clear the on-chip bitmap cache, which may help eliminate any residual rendering artifacts.

Best Regards,
BRT Community