How to access every pixel? - opengl

Question

If I have a PixelBuffer object of size (200 * 200 * 3) where each pixel has three consecutive spots for the RGB colors. How can I index them so that if i am trying to implement the DDA line drawing algorithm. I have seen a lot on the web that uses PutPixel(x,y) but im not sure how I can access the pixels in this method.

Answer 1

The pixels will be arranged row by row, with each pixel using 3 bytes. To address a point (x, y), you basically just need to multiply the y value by the size of a row (which is the width multiplied by 3), multiply the x value by the size of a pixel (3).
With a few constants for readability, the code for the function could look like this:
const int IMG_WIDTH = 200;
const int IMG_HEIGHT = 200;
const int BYTES_PER_PIXEL = 3;
const int BYTES_PER_ROW = IMG_WIDTH * BYTES_PER_PIXEL;
void PutPixel(uint8_t* pImgData, int x, int y, const uint8_t color[3])
{
uint8_t pPixel = pImgData + y * BYTES_PER_ROW + x * BYTES_PER_PIXEL;
for (int iByte = 0; iByte < BYTES_PER_PIXEL; ++iByte)
{
pPixel[iByte] = color[iByte];
}
}
Example how this function could be used:
// Allocate image data.
uint8_t* pImgData = new uint8_t[IMG_WIDTH * IMG_HEIGHT];
// Initialize image data, unless you are planning to set all pixels.
...
// Set pixel (50, 30) to yellow.
uint8_t yellow[3] = {255, 255, 0};
PutPixel(pImgData, 50, 30, yellow);
Once you have your image built in memory, you can store the content in a pixel buffer object using glBufferData():
GLuint bufId = 0;
glGenBuffers(1, &bufId);
glBindBuffer(GL_PIXEL_UNPACK_BUFFER, bufId);
glBufferData(GL_PIXEL_UNPACK_BUFFER, IMG_HEIGHT * BYTES_PER_ROW,
pImgData, GL_STATIC_DRAW);