opengl rendering pixels gives white lines

Question

I am trying to render an image by drawing it pixel for pixel (yes I know this isn't the most efficient but its more for learning purposes) and I got the weird problem that it draws white lines like this: the rendering

I use glut and visual studio 2013. This is my glutinit:

glutInitDisplayMode(GLUT_RGBA | GLUT_DOUBLE | GLUT_DEPTH | GLUT_ALPHA); // Double buffering, RGB format
glutInitWindowSize(width, heigth);
glutInitWindowPosition(0, 0);
glutCreateWindow("Test Plaatje");
glClearColor(1.0, 1.0, 1.0, 1.0);

glColor3f(1.0, 1.0, 1.0);

glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluOrtho2D(0.0, 1024, 0.0, 500);
glMatrixMode(GL_MODELVIEW);

and this is my rendering code with some variables explained:

...
std::vector<std::vector<RGB>> _map = std::vector<std::vector<RGB>>();
...
struct RGB
{
    int R;
    int G;
    int B;
};
...

void Map::draw()
{
    glBegin(GL_POINTS);

    for (float x = 0; x < _w; x++)
    {
        for (float y = 0; y < _h; y++)
        {
            RGB pixel = _map[y][x];
            glColor3ub(pixel.R, pixel.G, pixel.B);
            glVertex2f(_p[0] + x, _p[1] + y);
        }
    }

    glEnd();
}

I already tried shifting the pixels with some decimal value but that didn't work. I verified that the _map is correct by dumping the pixel values to file. for reference this is what it should render:

the source

I have no idea how to fix this. anyone have any ideas?

Try this: `glHint(GL_POINT_SMOOTH_HINT, GL_NICEST);` You probably want to enable blending beforehand: `glEnable(GL_BLEND); glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA); glEnable(GL_POINT_SMOOTH);` but I'm not sure which of those are necessary and which are not. Also try to increase point size `glPointSize(2.0)`. — n. m. could be an AI, Dec 30 '14 at 11:53
@n.m. i used glEnable(GL_BLEND); glEnable(GL_POINT_SMOOTH); glHint(GL_POINT_SMOOTH_HINT, GL_NICEST); and it worked! thankyou! — WillieWonka, Dec 30 '14 at 11:59
Assuming both your window and you image are exactly 1024x500 pixels (and the modelview matrix left at identity), your code should work if `_p[0]` and `_p[1]` are set to 0.5, without having to use `GL_POINT_SMOOTH`. — derhass, Dec 30 '14 at 13:39
@derhass yes but that varies with the configuration. the p makes sure that there is a top and side bar in the window. — WillieWonka, Dec 30 '14 at 14:29
@user2988415: can you elaborate on that? The thing is, you set up your projection matrix so that that the pixel centers lie at ".5", and the pixel borders are exactly at the integers. Ideally, you should always draw the points at pixel centers. But whatever you put in `_p` and in the modelview matrix, will also affect this. — derhass, Dec 30 '14 at 14:32
Why not upload the map data to a texture, bind it, and render a textured quad? — Colonel Thirty Two, Dec 30 '14 at 17:19

score 0 · Accepted Answer · answered Dec 30 '14 at 12:00

0

at n.m. tip i added glEnable(GL_BLEND); glEnable(GL_POINT_SMOOTH); glHint(GL_POINT_SMOOTH_HINT, GL_NICEST); at the end of my init and it worked!

answered Dec 30 '14 at 12:00

WillieWonka

115
1
11

Since @n.m. tip worked, I think it's better to let him/her post it as answer and accept it :) – Adri C.S. Dec 30 '14 at 12:12
@AdriC.S. i will when he does. i just thought it would be nice if the answer would already be clear. – WillieWonka Dec 30 '14 at 12:44
@AdriC.S. Credit is due to whoever posted a verified answer. I wasn't sure what exact configuration will work, and I'm still not sure this configuration is optimal. – n. m. could be an AI Dec 30 '14 at 12:50
@n.m It might not be optimal, but your solution worked. – Adri C.S. Dec 30 '14 at 13:05

score 0 · Answer 2 · answered Dec 31 '14 at 14:29

You can solve this floating point error in many ways:

use glPointSize(2.0)
use blend and point smooth
shift vertex coords to project in pixels centers +.5,+.5
use glRect instead of begin/vertex/end

Personalny i'd preffer vertex shift. But if you plan to zoom your image, better approach is glRect.

If you're looking for more serious solution, you should consider uploadind your pixels to texture and draw it with single quad. Also consider using glDrawPixels.

opengl rendering pixels gives white lines

2 Answers2