Skip to main content
Ebun.1
Ebun.1
Senior
March 25, 2020
Solved

from 24 to 16bit

  • March 25, 2020
  • 1 reply
  • 1223 views

Hello

I use STM32F769I-DISCO.

Does changing from 24bit to 16bit reduce the load on the CPU?

What is 16bit, if 24bit is 100%?

Also, can't TouchGFX change 24bit to 16bit?

Thank you.

This topic has been closed for replies.
Best answer by N. SANTINI

Hi,

In some case it may indeed reduce the CPU load but this is not what should be expected when changing the color format.

Changing from 24 bits to 16 bits means that each pixel of the frame buffer will use 2 bytes instead of 3 thus reducing the memory area needed to store the frame buffer.

It will also reduce the general memory bandwidth, notably on the FMC bus.

On the display side the LTDC will transfer less data to update the display (if the display supports 16 bits data).

The CPU is not involved at all in the process of refreshing the display so no difference.

If the display still requires 24 bits data then only the frame buffer will be in 16-bits.

On the rendering side (frame buffer update) the ChromArt hardware accelerator is used for most of the pixel operations (copy, copy with transparency and pixel format conversion) so you may observe a CPU drop only if you use transformations such as scaling or rotation for example.

Again if you replace you input images by 16 bits versions you will save FLASH memory, but not CPU usage since the ChromArt is able to access these data without the help of the CPU.

So in short you should mostly expect to reduce the memory usage and memory bandwidth when changing the color format from 24 to 16 bits.

Best regards,

Nicolas

1 reply

N. SANTINI
N. SANTINIBest answer
ST Employee
March 25, 2020

Hi,

In some case it may indeed reduce the CPU load but this is not what should be expected when changing the color format.

Changing from 24 bits to 16 bits means that each pixel of the frame buffer will use 2 bytes instead of 3 thus reducing the memory area needed to store the frame buffer.

It will also reduce the general memory bandwidth, notably on the FMC bus.

On the display side the LTDC will transfer less data to update the display (if the display supports 16 bits data).

The CPU is not involved at all in the process of refreshing the display so no difference.

If the display still requires 24 bits data then only the frame buffer will be in 16-bits.

On the rendering side (frame buffer update) the ChromArt hardware accelerator is used for most of the pixel operations (copy, copy with transparency and pixel format conversion) so you may observe a CPU drop only if you use transformations such as scaling or rotation for example.

Again if you replace you input images by 16 bits versions you will save FLASH memory, but not CPU usage since the ChromArt is able to access these data without the help of the CPU.

So in short you should mostly expect to reduce the memory usage and memory bandwidth when changing the color format from 24 to 16 bits.

Best regards,

Nicolas

Ebun.1
Ebun.1Author
Senior
March 26, 2020

Thank you for your reply.

For STM32F769I-DISCO, the memory uses SDRAM.

I think that 24bit is enough, so I thought there was no merit to 16bit.

Does this kind of idea look good?

Thank you.

N. SANTINI
N. SANTINI
ST Employee
March 26, 2020

Indeed, the main purpose of switching to 16bits is to reduce the frame buffer size in case you use only internal RAM that is usually quite small.

If external SDRAM is available then frame buffers are put there and memory usage becomes less critical as the need to switch from 24 to 16 bits.

Best regards,

Nicolas

Terms & ConditionsAccessibility statement