Posted on February 12, 2018 at 01:45
I've been working on a simple UI library for my project.
I'm using the stm32F7 with LTDC configured to RGB888 color space.
Whilst I was writing a color gradient function, I noticed that my gradients looked 'choppy'
Left gradient: Dithered RGB888 bitmap
Middle gradient: non-dithered RGB888 bitmap
Right gradient: custom gradient function
My gradient code does a simple loop:
for
(uint32_t
yPos = y;
yPos < height;
yPos++) {LCD
::fillRectangle(x,
yPos,
width,
1
,
LCD
::getGradientColor(yPos,
height,
fromColor,
toColor,
(ColorFormat
)hLtdcHandler
.LayerCfg
[0
].PixelFormat
));
}which calls the getGradientFunction:
uint32_t
LCD
::getGradientColor
(uint32_t
current,
uint32_t
max,
uint32_t
fromColor,
uint32_t
toColor) {float
position = (float
) current / max;
uint8_t
red = (uint8_t
) (((1.0
- position) * (fromColor &0xff0000
>>16
)) + (position * (toColor &0xff0000
>>16
)));
uint8_t
green = (uint8_t
) (((1.0
- position) * (fromColor &0x00ff00
>>8
)) + (position * (toColor &0x00ff00
>>8
)));
uint8_t
blue = (uint8_t
) (((1.0
- position) * (fromColor &0x0000ff
)) + (position * (toColor &0x0000ff
)));
switch
(hLtdcHandler
.LayerCfg
[0
].PixelFormat
) {case
LTDC_PIXEL_FORMAT_ARGB8888
:return
((uint8_t
)0xFF
<<24
) | (red <<16
) | (green <<8
) | blue;
default
:case
LTDC_PIXEL_FORMAT_RGB565
:case
LTDC_PIXEL_FORMAT_RGB888
:return
(red <<16
) | (green <<8
) | blue;
}}
Based on my calculations, I have a 600px high display and I am drawing a full-height gradient. I expect a different color value every ~2.35 lines (600 lines / 255 grey shades). However, my display shows a color change every ~9 lines.
Does anybody have an idea where I shall start looking for the cause?
I am using DMA to draw the two bitmaps, with the appropriate input color modes for both bitmaps.
For gradient I just use a simple register to memory call for each line.
