Thread: Can I have some Sample Code? (OpenGL, glDrawPixels();)

Posting this quote to show that I have looked, can anyone gimme some sample code that uses glDrawpixels()?


Preferably something that uses NeHe's base GL code... Really not interested in fudging with GLUT, seeing as how its not being used anymore...

14.015 How do I use glDrawPixels() and glReadPixels()?

glDrawPixels() and glReadPixels() write and read rectangular areas to and from the framebuffer, respectively. Also, you can access stencil and depth buffer information with the format parameter. Single pixels can be written or read by specifying width and height parameters of 1.

glDrawPixels() draws pixel data with the current raster position at the lower left corner. Problems using glDrawPixels() typically occur because the raster position is set incorrectly. When the raster position is set with the glRasterPos*() function, it is transformed as if it were a 3D vertex. Then the glDrawPixels() data is written to the resulting device coordinate raster position. (This allows you to tie pixel arrays and bitmap data to positions in 3D space).

When the raster position is outside the view volume, it's clipped and the glDrawPixels() call isn't rendered. This occurs even when part of the glDrawPixels() data would be visible. Here's info on how to render when the raster position is clipped.

glReadPixels() doesn't use the raster position. Instead, it obtains its (X,Y) device coordinate address from its first two parameters. Like glDrawPixels(), the area read has x and y for the lower left corner. Problems can occur when reading pixels if:

The area being read is from a window that is overlapped or partially offscreen. glReadPixels() will return undefined data for the obscured area. (More info.)
Memory wasn't allocated for the return data (the 7th parameter is a NULL pointer) causing a segmentation fault, core dump, or program termination. If you think you've allocated enough memory, but you still run into this problem, try doubling the amount of memory you've allocated. If this causes your read to succeed, chances are you've miscalculated the amount of memory needed.
For both glDrawPixels() and glReadPixels(), keep in mind:

The width and height parameters are in pixels.
If the drawn or read pixel data seems correct, but is slightly off, make sure you've set alignment correctly. Argument values are controlled with the glPixelStore*() functions. The PACK and UNPACK values control sending and receiving pixel data, from and to OpenGL, respectively.

Originally Posted by Shamino

Ultimately, what i need to do, is change a certain set of pixels (20x20) green, which arent outlined by a polygon of any sort....without killing my system... I really don't know where to look anymore

So... you know exactly what pixels you need to change? If so, then change to a 2D ortho perspective with Width and Height the same as the window and draw a single poly untextured 20x20.

If you want a way to optimize the change from 2D and 3D I can help on that.

Are they all the same size? You could use a DisplayList to create one, and then just repeat it. That should be fast enough.

Code:

unsigned int ID;  //this is a unique ID for the list, like a texture ID
ID = glGenLists(1);
glNewList(ID, GL_COMPILE);
   glBegin(GL_QUADS);
   //Draw your model here
   glEnd();
glEndList();

//------------------------------------------------

//Anywhere you want to draw the above object, just type

glCallList(ID);

A display list creates a cache of any commands you insert inside of this list so the can be executed quickly. Vertex arrays are on par with Display Lists as far as speed goes, vertex arrays allow you to alter the data during the program, you have to delete the display list and remake it to alter the data there. The tradeoff for this reduced flexability is that display lists are very simple.