Drawing large numbers of pixels in OpenGL

I'开发者_运维技巧ve been working on some sound processing code and now I'm doing some visualizations. I finished making a spectrogram spectrogram, but how I am drawing it is too slow.

I'm using OpenGL to do 2D drawing, which has made searching for help more difficult. Also I am very new to OpenGL, so I don't know the standard way things are done.

I am storing the r,g,b values for each pixel in a large matrix. Each time I get a small sound segment, I process it and convert it to column of pixels. Everything is shifted to the left 1 pixel, and the new line is put at the end.

Each time I redraw, I am looping through setting the color and drawing each pixel individually, which seems like a horribly inefficient way to do this.

Is there a better way to do this? Is there some method for simply shifting a bunch of pixels over?

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They are many ways to improve your drawing speed. The simplest would be to allocate a an RGB texture that you will draw using a screen aligned texture quad. Each time that you want to draw a new line you can use glTexSubImage2d to a load a new subset of the texture and then you redraw the quad.

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Are you perhaps passing a lot more data to the graphics card than you have pixels? This could happen if your FFT size is much larger than the height of the drawing area or the number of spectral lines is a lot more than its width. If so, it's possible that the bottle neck could be passing too much data across the bus. Try reducing the number of spectral lines by either averaging them or picking (taking the maximum in each bin for a set of consecutive lines).

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GL_POINTS, VBO, GL_STREAM_DRAW.

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I know this is an old question, but . . .

Use a circular buffer to store the pixels, and then simply call glDrawPixels twice with the appropriate offsets. Something like this untested C:

#define SIZE_X 800
#define SIZE_Y 600
unsigned char pixels[SIZE_Y][SIZE_X*2][3];
int start = 0;

void add_line(const unsigned char line[SIZE_Y][1][3]) {
    int i,j,coord=(start+SIZE_X)%(2*SIZE_X);
    for (i=0;i<SIZE_Y;++i) for (j=0;j<3;++j) pixels[i][coord][j] = line[i][0][j];
    start = (start+1) % (2*SIZE_X);
}
void draw(void) {
    int w;
    w = 2*SIZE_X-start;
    if (w!=0) glDrawPixels(w,SIZE_Y,GL_RGB,GL_UNSIGNED_BYTE,3*sizeof(unsigned char)*SIZE_Y*start+pixels);
    w = SIZE_X - w;
    if (w!=0) glDrawPixels(SIZE_X,SIZE_Y,GL_RGB,GL_UNSIGNED_BYTE,pixels);
}