I am trying to create a dynamic plotter in SDL2 for an embedded project. Currently, the code executes on both architectures, x86, and ARM. On x86 I get a smooth running plotter, however, on ARM it runs very slow at a fraction of the frames I get on x86. I am pretty sure this is because I rerender every pixel on the surface at this is a massive overheat on the embedded device.
I tried rendering the new content to a texture, copy it to the renderer and then render, but this did not work at all.
Due to the double-buffering, I have to clear every frame. Otherwise, I will "drop" changes. But I also need to render the old data points and only overwrite them when the plotter reaches them again.
Is there a way in SDL2 to save these data points to some sort of canvas and only add (redraw) the newly added ones?
Heres my source code:
Plotter.cpp
#include <SDL2/SDL.h>
#include <stdio.h>
#include "PlotterHelper.h"
/*Implementation*/
int main(int arhc, char * argv[])
{
//Top and bottom viewport
SDL_Rect topViewport;
SDL_Rect bottomViewport;
topViewport = CreateViewPort(0,0, SCREEN_WIDTH, (SCREEN_HEIGHT/2));
bottomViewport = CreateViewPort(0,(SCREEN_HEIGHT/2), SCREEN_WIDTH, SCREEN_HEIGHT/2);
float timeFrame = 4.0;
int updateWidth = round(SCREEN_WIDTH/(60.0 * timeFrame));
uint8_t backgroundColor = 0xff;
int delayTime = 0;
int sinusScale = 0;
int rectWidth = RECT_WIDTH;
bool rectEnd = false;
SDL_Point points[SCREEN_WIDTH] = {0,0};
int pointPosition = 0;
if (!init())
{
printf("Init failed!\n");
}
else
{
SDL_ShowCursor(SDL_DISABLE);
//Main lopp flag
bool quit = false;
//Event handler
SDL_Event event;
//While application is running
while(!quit)
{
//Handle events on queue
while (SDL_PollEvent( &event) != 0)
{
//User requests quit
if(event.type == SDL_QUIT)
{
quit = true;
}
else if(event.type == SDL_KEYDOWN)
{
switch(event.key.keysym.sym)
{
case SDLK_w:
delayTime += 50;
if(delayTime > 5000)
delayTime = 5000;
break;
case SDLK_s:
delayTime -= 50;
if(delayTime < 0)
delayTime = 0;
break;
case SDLK_d:
timeFrame -= 1;
if(timeFrame < 1)
timeFrame = 1.0;
updateWidth = round(SCREEN_WIDTH/(60.0 * timeFrame));
printf("timeFrame = %lf\n", timeFrame);
break;
case SDLK_a:
timeFrame += 1;
if(timeFrame > 44)
timeFrame = 44.0;
updateWidth = round(SCREEN_WIDTH/(60.0 * timeFrame));
printf("timeFrame = %lf\n", timeFrame);
break;
case SDLK_r:
if(backgroundColor == 0x3f)
break;
else
{
++backgroundColor;
break;
}
case SDLK_f:
if(backgroundColor == 0x00)
break;
else
{
--backgroundColor;
break;
}
}
}
}
//Reset Plotter when the end of the window was reached
if(pointPosition > SCREEN_WIDTH-1)
{
pointPosition = 0;
sinusScale = (rand() % 100 + 1) - 50;
rectWidth = RECT_WIDTH;
rectEnd = false;
}
//Handler eraser when he reaches end of window
if(((SCREEN_WIDTH-1) - pointPosition) < RECT_WIDTH)
{
rectWidth = (SCREEN_WIDTH -1) - pointPosition;
rectEnd = true;
}
//Clear screen
SDL_SetRenderDrawColor(gRenderer, backgroundColor, backgroundColor, backgroundColor, 0xff);
SDL_RenderClear(gRenderer);
//Draw top viewport
SDL_RenderSetViewport( gRenderer, &topViewport );
SDL_SetRenderDrawColor(gRenderer, 0x00, 0x66, 0x00, 0xff);
for(int iterator = 0; iterator <= SCREEN_WIDTH -1; ++iterator)
{
SDL_RenderDrawLine(
gRenderer,
points[iterator].x,
0,
points[iterator].x,
SCREEN_HEIGHT/2);
}
PlotEraser(rectEnd, backgroundColor, rectWidth,points[pointPosition].x );
//raw bottom viewport
SDL_RenderSetViewport( gRenderer, &bottomViewport );
SDL_SetRenderDrawColor(gRenderer, 0x00, 0x66, 0x00, 0xff);
for(int iterator = 0; iterator <= SCREEN_WIDTH -1; ++iterator)
{
SDL_RenderDrawLine(
gRenderer,
points[iterator].x,
SCREEN_HEIGHT/2,
points[iterator].x,
points[iterator].y);
}
PlotEraser(rectEnd, backgroundColor, rectWidth,points[pointPosition].x );
for(int iterator = pointPosition; iterator <= pointPosition + updateWidth; ++iterator)
{
points[iterator].x = iterator;
points[iterator].y = round(((SCREEN_HEIGHT/4 )* sin(iterator/(100.0+sinusScale))) + SCREEN_HEIGHT/4);
}
pointPosition += updateWidth;
//Update Screen
SDL_RenderPresent(gRenderer);
SDL_Delay(delayTime);
}
}
//Free resources and close SDL
close();
return 0;
}
/*End of File*/
PlotterHelper.cpp
/*Includes*/
#include "PlotterHelper.h"
SDL_Window * gWindow = NULL;
SDL_Renderer * gRenderer = NULL;
SDL_Renderer * intermediate = NULL;
/*Implementation*/
/*********************************************************************
*********************************************************************
*********************************************************************
*/
bool init()
{
//Init flag
bool success = true;
//Init SDL
if(SDL_Init(SDL_INIT_VIDEO) < 0)
{
printf( "SDL could not initialize! SDL_Error: %s\n", SDL_GetError() );
success = false;
}
else
{
//Set texture filtering to linear
if( !SDL_SetHint( SDL_HINT_RENDER_SCALE_QUALITY, "1" ) )
{
printf( "Warning: Linear texture filtering not enabled!" );
}
//Set VSYNC
if( !SDL_SetHint( SDL_HINT_RENDER_VSYNC, "1" ) )
{
printf( "Warning: VSYNC not enabled!" );
}
//Create window
gWindow = SDL_CreateWindow(
"SDL Plotter",
SDL_WINDOWPOS_UNDEFINED,
SDL_WINDOWPOS_UNDEFINED,
SCREEN_WIDTH,
SCREEN_HEIGHT,
SDL_WINDOW_SHOWN );
if (NULL == gWindow)
{
printf( "Window could not be created! SDL_Error: %s\n", SDL_GetError() );
success = false;
}
else
{
//Create renderer for window
gRenderer = SDL_CreateRenderer(gWindow, -1, SDL_RENDERER_ACCELERATED );
if(NULL == gRenderer)
{
printf("Renderer could not be created! SDLError: %s\n", SDL_GetError());
success = false;
}
else
{
//Initialise renderer colour
SDL_SetRenderDrawColor(gRenderer, 0xff, 0xff, 0xff, 0xff);
}
}
}
return success;
}
/*********************************************************************
*********************************************************************
*********************************************************************
*/
void close()
{
//Destroy window
SDL_DestroyRenderer(gRenderer);
SDL_DestroyWindow(gWindow);
gRenderer = NULL;
gWindow = NULL;
//Quit SDL subsystems
SDL_Quit();
}
/*********************************************************************
*********************************************************************
*********************************************************************
*/
SDL_Rect CreateViewPort(int x, int y, int w, int h)
{
SDL_Rect Viewport;
Viewport.x = x;
Viewport.y = y;
Viewport.w = w;
Viewport.h = h;
return Viewport;
}
/*********************************************************************
*********************************************************************
*********************************************************************
*/
void PlotEraser(bool rectEnd, uint8_t backgroundColor, int rectWidth, int x)
{
SDL_Rect fillRectBot = {x, 0, rectWidth, SCREEN_HEIGHT/2};
SDL_SetRenderDrawColor(gRenderer, backgroundColor, backgroundColor, backgroundColor, 0xff);
SDL_RenderFillRect(gRenderer, &fillRectBot);
if(rectEnd)
{
int startRecWidth = RECT_WIDTH - rectWidth;
SDL_Rect startRec = {0, 0, startRecWidth, SCREEN_HEIGHT/2};
SDL_RenderFillRect(gRenderer, &startRec);
}
}
/*End of File*/
PlotterHelper.h
#ifndef PLOTTER_HELPER_H_
#define PLOTTER_HELPER_H_
#include <SDL2/SDL.h>
#include <stdio.h>
#include <cmath>
#include <string>
/*Globals*/
//Screen constants
const int SCREEN_WIDTH = 1024;
const int SCREEN_HEIGHT = 768;
const int RECT_WIDTH = 10;
//The window we'll be rendering to
extern SDL_Window * gWindow;
//The window renderer
extern SDL_Renderer * gRenderer;
extern SDL_Renderer * intermediate;
/*Prototypes*/
/*********************************************************************
*********************************************************************
*********************************************************************
*/
//Starts up SDL and creates window
bool init();
/*********************************************************************
*********************************************************************
*********************************************************************
*/
//Free media and shut down SDL
void close();
/*********************************************************************
*********************************************************************
*********************************************************************
*/
//CreateVieport
SDL_Rect CreateViewPort(int x, int y, int w, int h);
/*********************************************************************
*********************************************************************
*********************************************************************
*/
//Plot Eraser
void PlotEraser(bool rectEnd, uint8_t backgroundColor, int rectWidth, int x);
#endif /* PLOTTER_HELPER_H_ */
/*End of File*/
You probably have a couple of options that can help:
You ought to be able to accomplish this using a "render target texture". That is an SDL_Texture you create (see SDL_CreateTexture) with the SDL_TEXTUREACCESS_TARGET flag.
You would draw new points to this texture by calling SDL_SetRenderTarget and passing this texture before then rendering the points.
your main loop would then need to call SDL_SetRenderTarget and pass nullptr to restore the window as the rendering target. You then render your texture to the window every frame.
The SDL documentation has a small sample demoing how to use a render target texture.
the latest version of SDL 2 supports render batching if you set SDL_HINT_RENDER_BATCHING (SDL_SetHint(SDL_HINT_RENDER_BATCHING, "1");). You should use SDL_CreateWindowAndRenderer() to ensure you have batching enabled.
This may not do anything if your platform doesnt support hardware acceleration. You can use this in conjunction with a render target. read about SDL render batching here.
This is separate from render batching and should work on any version of SDL 2.
If you can, collect all your rectangles/points together in an array you can use
SDL_RenderDrawPoints or SDL_RenderDrawRects to have SDL do them all at once which should improve performance even if you have no hardware acceleration. This can be used in conjunction with a render target.
If your device lacks hardware acceleration, then it might be faster to ditch SDL_Renderer and instead use SDL_GetWindowSurface to get the SDL_Surface for the window and use SDL_BlitSurface (or manually set pixels via surface->pixels) to draw directly to that then update the window with SDL_UpdateWindowSurface. See also SDL_UpdateWindowSurfaceRects for how to only update the rectangles you are changing for even better performance.
You will need to familiarize yourself with SDL_Surface as you will need to inspect the pixel format of the window surface in order to update it correctly should you choose to directly manipulate the pixels.