Banding in Bitmaps

Question

Hi,

We're working on a STM32F469 with RGB888 and are seeing a lot of banding on bitmaps on the target:

 

The PNG files are smooth. We're using compression, but I am not convinced EmWi is causing this banding with its compression.

 

 

Answer 1

HI,

how did you configure your images? Please check the format of your bitmap resource and ensure that it is set to native and not to RGB565.

How does it appear within the prototyper?

Let me know....

Best regards,

Manfred.

Comments

Hi Manfred,

Bitmaps are all set to Native, and in the prototyper (And our windows application) they look perfect.

Regards

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HI,

ok, so the framebuffer content seems to be correctly.

Is the LTDC configured to use 8 bits per color component (R/G/B)?

In case you do not use DSI: Is your display connected with 24 bits (8 bits per color component) or only with 18 bits?

And finally, is you display able to show 2^24 colors?

It seems to me, that the color information gots somehow lost on the way from the framebuffer to the display...

Best regards,

Manfred.

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Hi Manfred,

Thanks for you answer. You are right, colour is going missing over the link - we're looking at the DSI/LTDC configuration now.

Regards

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Latest info I have is that they *think* the controller's DSI link only supports 18 bit pixels. (RGB666)

I've been asked to ask you if there's any way of minimising this banding, but I'm not sure there is.

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If your bitmap does not contain any transparency, you can try to set the Format of the bitmap resource to RGB565 and use Dithering in order to get some error distribution.

Does this improve the resulting appearance?

Best regards,

Manfred.

 

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Hi Manfred,

I tried this last week and it does improve the images. Although doesn't this mean we are taking a 888 image, converting it to 565, rendering it on a frame buffer that's 888 and then transferring it to a display that's using 666.

Thanks for all your help.

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Hi,

yes, the story is exactly as you described it. In case that Embedded Wizard Studio does the color reduction, the advantage is the dithering. Otherwise, the lower bits are just ignored.

In case that a display is able to show (or connected with) 18bits - it is worth to consider to use RGB565 as framebuffer format instead of RGB888. This requires less memory, less memory bandwidth and improves the graphics performance.

Best regards,

Manfred.