How to render a rectangle SDL2 texture from a buffer of hex values? - c++

Question

I'm having some trouble understanding texture streaming and loading a 2D texture from an array of raw pixel data.
Here is my code:
#include <stdio.h>
#include <stdlib.h>
#include <SDL2/SDL.h>
#define WINDOW_W 640
#define WINDOW_H 320
int main(int argc, char *argv[])
{
if (SDL_Init(SDL_INIT_EVERYTHING))
exit(1);
/* Set up main window. */
SDL_Window *window = SDL_CreateWindow(
"Texture Streaming",
SDL_WINDOWPOS_CENTERED,
SDL_WINDOWPOS_CENTERED,
WINDOW_W,
WINDOW_H,
SDL_WINDOW_SHOWN
);
if (window == NULL)
exit(2);
/* Set up renderer. */
SDL_Renderer *renderer = SDL_CreateRenderer(
window,
-1,
SDL_RENDERER_ACCELERATED | SDL_RENDERER_PRESENTVSYNC
);
if (renderer == NULL)
exit(3);
SDL_Event ev;
int window_running = 1;
SDL_Texture *texture = SDL_CreateTexture(renderer, SDL_PIXELFORMAT_RGBA32, SDL_TEXTUREACCESS_STATIC, 64, 32);
Uint32 pixels[64 * 32];
for (int i = 0; i < (64 * 32); i++)
pixels[i] = 0xFF00FFFF;
for (int i = 0; i < 64; i++)
pixels[i] = 0xFF0000FF;
SDL_UpdateTexture(texture, NULL, pixels, 4);
while (window_running)
{
while (SDL_PollEvent(&ev) != 0)
{
if (ev.type == SDL_QUIT)
window_running = 0;
}
SDL_SetRenderDrawColor(renderer, 0xFF, 0xFF, 0xFF, 0xFF);
SDL_RenderClear(renderer);
SDL_RenderCopy(renderer, texture, NULL, NULL);
SDL_RenderPresent(renderer);
}
SDL_DestroyRenderer(renderer);
SDL_DestroyWindow(window);
SDL_Quit();
return 0;
}
Instead of my program drawing the first 64 pixels red and the rest magenta, it just spits out a random chunk of red and yellow pixels.
I'm having trouble understanding SDL_CreateTexture and SDL_UpdateTexture.

Answer 1

Your pitch in UpdateTexture is wrong. Pitch is byte length of a row, not single pixel. In your case pitch is 64*4.
Pixel format SDL_PIXELFORMAT_RGBA32 is alias to either SDL_PIXELFORMAT_RGBA8888 or SDL_PIXELFORMAT_ABGR8888 (latter in your case). Look at https://wiki.libsdl.org/SDL_PixelFormatEnum . So your 0xFF00FFFF is yellow (R=0xff, G=0xff, B=0, A=0xff).
Finally, if we're talking about texture streaming (i.e. updating every frame or otherwise very often), you should create streaming texture and use LockTexture/UnlockTexture. UpdateTexture is for static textures.