基于stm32f4的三維旋轉(zhuǎn)顯示平臺(tái)

現(xiàn)實(shí)的世界是一個(gè)擁有寬度、高度和深度的三維立體世界。在平面二維顯示技術(shù)已經(jīng)成熟的今天，三維立體顯示技術(shù)首當(dāng)其沖的成為了當(dāng)今顯示技術(shù)領(lǐng)域的研究熱點(diǎn)。

本作品搭建了基于stm32f4的三維旋轉(zhuǎn)顯示平臺(tái)，它的顯示原理屬于三維顯示中的體三維顯示一類(lèi)。它是通過(guò)適當(dāng)方式來(lái)激勵(lì)位于透明顯示體內(nèi)的物質(zhì)，利用可見(jiàn)輻射光的產(chǎn)生三維體像素。當(dāng)體積內(nèi)許多方位的物質(zhì)都被激勵(lì)后，便能形成由許多分散的體像素在三維空間內(nèi)構(gòu)成三維圖像。

體三維顯示又稱為真三維顯示，因?yàn)樗尸F(xiàn)的圖像在真實(shí)的三維空間中，展示一個(gè)最接近真實(shí)物體的立體畫(huà)面，可同時(shí)允許多人，多角度裸眼觀看場(chǎng)景，無(wú)序任何輔助眼鏡。

本作品的特點(diǎn)在于，利用stm32f4的浮點(diǎn)運(yùn)算能力，實(shí)現(xiàn)了低成本的體三維顯示數(shù)據(jù)的生產(chǎn)，并利用類(lèi)似分布式處理的系統(tǒng)結(jié)構(gòu)，滿足了體三維顯示所需要的巨大數(shù)據(jù)吞吐量，等效吞吐量可達(dá)約300Mb/s

1.系統(tǒng)方案

眾所周知人眼在接收被觀察物體的信息時(shí)，攜帶物體信息的光信號(hào)通過(guò)人眼細(xì)胞及神經(jīng)傳入大腦神經(jīng)，光的作用時(shí)間只是一個(gè)很短暫的時(shí)間段，當(dāng)光的作用時(shí)間結(jié)束后，視覺(jué)影像并不會(huì)立即消失，這種殘留的視覺(jué)稱“后像”，視覺(jué)的這一現(xiàn)象則被稱為“視覺(jué)暫留”（ duration of vision）物體在快速運(yùn)動(dòng)時(shí), 當(dāng)人眼所看到的影像消失后，人眼仍能繼續(xù)保留其影像0.1---0.4秒左右的圖像。在空間立體物體離散化的基礎(chǔ)上，再利用人眼視覺(jué)暫留效應(yīng)，基于LED陣列的“三維體掃描”立體顯示系統(tǒng)便實(shí)現(xiàn)了立體顯示效果。如圖1所示，以顯示一個(gè)空心的邊長(zhǎng)為單位1的正方體為例。LED顯示陣列組成的二維顯示屏即為正方體每個(gè)離散平面的顯示載體，LED顯示屏上的被點(diǎn)亮的LED即為正方體的平面離散像素。我們將該LED顯示平面置于軸對(duì)稱的角度機(jī)械掃描架構(gòu)內(nèi)，在嚴(yán)格機(jī)電同步的立體柱空間內(nèi)進(jìn)行各離散像素的尋址、賦值和激勵(lì)，由于機(jī)械掃描速度足夠快，便在人眼視覺(jué)上形成一個(gè)完整的正方體圖像。圖1(a)所示為立方體0度平面二維切片圖，圖1(b)所示為立方體45度平面二維切片圖，圖1(c)所示為立方體135度平面二維切片圖，圖1(d)所示為立方體180度平面二維切片圖，圖1(e)所示為觀看者觀察到的完整立方體顯示效果。

圖1

系統(tǒng)方案如圖2所示，整個(gè)系統(tǒng)由四個(gè)模塊組成，其中數(shù)據(jù)獲取單元主要由在PC上的上位機(jī)完成，利用3D-Max，OpenCV，OpenGL，將三維建模數(shù)據(jù)轉(zhuǎn)化成三維矢量表述文件，傳給由STM32F4 Discovery開(kāi)發(fā)板構(gòu)成的控制單元，利用其上的角度傳感器，結(jié)合wifi模塊或以太網(wǎng)模塊通過(guò)電力線模式傳給LED旋轉(zhuǎn)屏單元，其中的STM32F4負(fù)責(zé)將ASE文件解析成LED顯示陣列所需的點(diǎn)云數(shù)據(jù)流，通過(guò)串行總線傳輸給由FPGA驅(qū)動(dòng)的LED顯示陣列，通過(guò)LED刷新速率與機(jī)械單元旋轉(zhuǎn)速率相匹配，從而實(shí)現(xiàn)體三維顯示的效果。

圖2

2.系統(tǒng)硬件設(shè)計(jì)

系統(tǒng)的機(jī)械部分如圖3所示，顯示面板的硬件結(jié)構(gòu)如圖4，圖5所示。本系統(tǒng)的底部是直流電機(jī)和碳刷，直流電機(jī)主要負(fù)責(zé)帶動(dòng)上層的顯示屏幕高速旋轉(zhuǎn)，而碳刷則負(fù)責(zé)傳遞能量和通信信號(hào)。在顯示屏幕的正面是由96*128構(gòu)成的三色LED點(diǎn)陣，F(xiàn)PGA的PWM信號(hào)通過(guò)驅(qū)動(dòng)芯片控制三色LED從而實(shí)現(xiàn)真彩顯示。在屏幕背面由多塊STM32F4，SD卡，F(xiàn)IFO構(gòu)成，主要負(fù)責(zé)解析由控制單元傳過(guò)來(lái)的ASE文件，并實(shí)時(shí)生成體三維顯示數(shù)據(jù)，并傳給LED燈板的驅(qū)動(dòng)FPGA，并通過(guò)其實(shí)現(xiàn)最終的圖像顯示。

圖3

圖4

圖5

3.系統(tǒng)軟件設(shè)計(jì)

3.1軟件控制流程：

3.2關(guān)于實(shí)時(shí)生成體三維顯示數(shù)據(jù)的討論：

一個(gè)瓦片64*32

LED層FPGA*8：每個(gè)16*16LED

中間層stm32*2：每個(gè)4LED層的FPGA，也即32*32

由于經(jīng)過(guò)壓縮，一個(gè)led數(shù)據(jù)為4bits

所以一個(gè)stm32每一幀所要生成的數(shù)據(jù)為32*32*0.5bytes = 512bytes

轉(zhuǎn)速800轉(zhuǎn)，一幀1/800s = 1.25ms = 1250000ns

stm32f4主頻168Mhz，指令周期 = 5.93ns

約可執(zhí)行20萬(wàn)多條指令

假設(shè)fsmc總線的速度為50Mhz，則每幀寫(xiě)入的時(shí)間大概在0.02ms內(nèi)

程序總體思路

事先算出所有電子幀上非零的點(diǎn)，以及連續(xù)0的個(gè)數(shù)，在每一個(gè)電子幀同步后，算出生成下一幀的數(shù)據(jù)，寫(xiě)入fifo

輸入：線段端點(diǎn)的集合

//input: endpoints of segments which formed the outline of a 3D model

//x position with range 0-95

//y position with range 0-95

//z position with range 0-128

/******************************************/

//from later discussion, one of the Q format

//type should replace the char type

/******************************************/

struct Coordinate_3D

{

_iq xPosition;

_iq yPosition;

_iq zPosition;

};

//after you get the intersection points in 3d coordinate, you need to remap it into 2d coordinate on the very electrical plane,

//and the conversion is quite simple Coordinate_2D.yPosition = Coordinate_3D.zPosition; Coordinate_2D.xPosition = sqrt(xPosition^2+yPosition^2)

struct Coordinate_2D

{

char xPosition;

char yPosition;

};

struct Line

{

struct Coordinate_3D beginPoint;

struct Coordinate_3D endPoint;

unsigned char color;

};

//frame structure to store the visible points in one electrical frame

//need to be discussed

//here's the prototype of the Frame structure, and basically the frame struture should contain the visible points,

//and the zero points. As we have enclosed the number of zero points after each visible points in their own data structure,

//only the number of zero points at the beginning of the whole frame should be enclosed in the frame struture

struct Frame

{

int zerosBefore;

PointQueue_t visiblePointQueue;  

};

//we need a union structure like color plane with bit fields to store the color imformation of every four FPGAs in one data segment

//actually, it's a kind of frustrateing thing that we had to rebind the data into such an odd form.

union  ColorPalette

{

struct

{

unsigned char color1 : 4;

unsigned char color2 : 4;

unsigned char color3 : 4;

unsigned char color3 : 4;

}distributedColor;

unsigned short unionColor;

};

//and now we need a complete point structure to sotre all the imformation above

//here we add a weight field = yPosition*96 + xPosition, which will facilitate

//our sort and calculation of the zero points number between each visible point

//it's important to understand that, 4 corresponding points on the LED panel

//will share one visiblepoint data structure.(一塊stm32負(fù)責(zé)4塊16*16的LED，每塊對(duì)應(yīng)的點(diǎn)的4位顏色信息，拼成16位的數(shù)據(jù)段)

struct VisiblePoint

{

struct Coordinate_2D coord;

union Colorplane ColorPalette;

int weight;

int zerosAfter;

};

//as now you can see, we need some thing to store the visible points array

typedef struct QueueNode

{

struct VisiblePoint pointData;

struct QueueNode * nextNode;

}QueueNode_t, *QueueNode_ptr;

typedef struct

{

QueueNode_ptr front;

QueueNode_ptr rear;

}PointQueue_t;

//finally, we will have 16*16 words(16 bits)to write into the fifo after each electrial frame sync cmd.

//it may hard for us to decide the frame structure now, let's see how will the work flow of the algorithm be.

//firstly, the overall function will be like this

void Real3DExt(struct Line inputLines[], int lineNumber, struct Frame outputFrames[])

//then we need some real implementation function to calculate the intersection points

//with 0 = no intersection points, 1 = only have one intersection points, 2 = the input line coincides the given electrical plane

//2 need to be treated as an exception

//the range of the degree is 0-359

//it's important to mention that each intersection point we calculate, we need to

//remap its coordinate from a 32*32 field to x,y = 0-15, as each stm32 only have a 32*32

//effective field(those intersection points out of this range belong to other stm32), which can be decided by its address

int InterCal(struct Line inputLine, struct VisiblePoint * outputPoint, int degree)

//so we will need something like this in the Real3DExt function:

for (int j = 0; j < 360; j++)

{

for(int i = 0; i < lineNumber; i++ )

InterCal(struct Line inputLine, struct VisiblePoint outputPoint, int degree);

......

}

/******************************************/

//simple float format version of InterCal

/******************************************/

//calculate formula

//Q = [-1,1,-1];

//P = [1,1,-1];

//V = Q - p = [-2,0,0];

//Theta = pi/6;

//Tmp0 = Q(1)*sin(Theta) - Q(2)*cos(Theta);

//Tmp1 = V(1)*sin(Theta) - V(2)*cos(Theta);

//Result = Q - (Tmp0/Tmp1)*V

float32_t f32_point0[3] = {-1.0f,1.0f,-1.0f};

float32_t f32_point1[3] = {1.0f,1.0f,-1.0f};

float32_t f32_directionVector[3], f32_normalVector[3], f32_theta,

f32_tmp0, f32_tmp1, f32_tmp2, f32_result[3];

arm_sub_f32(f32_point0,f32_point1,f32_directionVector,3);

f32_theta = PI/6.0f;

f32_normalVector[0] = arm_sin_f32(f32_theta);      

f32_normalVector[1] = arm_cos_f32(f32_theta);

f32_normalVector[2] = 0.0f;

arm_dot_prod_f32(f32_point0, f32_normalVector, 3, &f32_tmp0);

arm_dot_prod_f32(f32_directionVector, f32_normalVector, 3, &f32_tmp1);

f32_tmp2 = f32_tmp0/f32_tmp1;

arm_scale_f32(f32_normalVector, f32_tmp2, f32_normalVector, 3);

arm_sub_f32(f32_point0, f32_normalVector, f32_result, 3);

//and than we need to decide whether to add a new visible point in the point queue, or to update

//the color field of a given point in the point queue(as 4 visible point share one data structure). from this point, you will find that, it may be

//sensible for you not to diretly insert a new point into the end of point queue but to insert it in order

//when you build the pointqueue. it seems more effective.

void EnPointQueue(PointQueue_t * inputQueue, QueueNode_t inputNode);

//finally we will get an sorted queue at the end of the inner for loop

//than we need to calculate the number of invisible points between these visible points

//and to store it in each frame structure. the main purpose to do so is to offer an quick generation

//of the blank point(color field = 16'b0) between each electrical frame

//the work flow will be like this:

loop

{

dma output of the blank points;

output of the visible points;

}

/******************************************/

//some points need more detailed discussion

/******************************************/

//1.memory allocation strategy

//a quite straight forward method will be establishing a big memnory pool in advance, but the drawback of this method

//is that it's hard for you to decide the size of the memory pool. Another way would be the C runtime library method,

// and you can use build-in function malloc to allocate the memory, but it will be a quite heavy load for the m3 cpu

// as you need dynamic memeory allocation throughout the algorithm.

//2.the choice of Q format of the IQMATH library

//from the discussion above, the range of the coordnate is about 1-100, but the range of sin&cos is only 0-1,so there's a large gap between them.

//may be we can choose iq24?? Simultaneously, another big problem will be the choice between IQMATH and arm dsp library as their q format is

//incompatible with each other. as far as my knowledge is concerned, we should choose IQMATH with m3 without fpu, and cmsis dsp library with m4 with fpu.

//more detail discussion about the range of the algorithm

//x,y range is -64 to 64

//the formula is

//Tmp0 = Q(1)*sin(Theta) - Q(2)*cos(Theta);

//Tmp0 range is -128 to 128

//Tmp1 = V(1)*sin(Theta) - V(2)*cos(Theta);

//Tmp1 range is -128 to 128

//Result = Q - (Tmp1/Tmp2)*V

//because the minimal precision of the coordinate is 1, so if the result of Tmp1/Tmp2 is bigger than 128, the Result will be

//saturated. With the same reson, if (Tmp1/Tmp2)*V >= 128 or <= -127, the result will be saturated

4.系統(tǒng)創(chuàng)新