CODEWITHSUNDEEP
mathopengl
Cheiron
Cheiron

Reputation: 3736

OpenGL Vertical Flip: -360 + -360 = 0?

I am working with OpenGL and I wanted to invert the image. So I went here, asked a question and finally I had the following code:

    glMatrixMode(GL_PROJECTION);
    glScalef(-1,1,1);
    glTranslatef(-width(),0,0);

From what I understand from this, the position of every pixel gets inverted, so the pixels that were on the right of the image are now on the same absolute position, but are the left of the image, so I have to move the entire thing back exactly as many pixels as its wide: 360 (which is the size of the "canvas", so in the snippents the function width() is being used)! So to undo this process I would invert the image again and then move it back to where it came from:

    glMatrixMode(GL_PROJECTION);
    glScalef(-1,1,1);
    glTranslatef(width(),0,0);

Nope, blackscreen. I have to do exactly the same thing twice to undo the flipping: I have to move with -360 every time I flip the image. Why?

Upvotes: 1

Views: 265

Answers (1)

Nico Schertler
Nico Schertler

Reputation: 32587

It's exactly as Daniel Fischer mentioned in the comment. Here is an illustration of the process.

What you must have in mind is that the transformations operate on the transformed coordinate systems.

We start with the image (grey) on the screen (green):

F1

Then we scale the image. So the origin is preserved, but the x-axis is mirrored.

F2

No we have to move the image onto the screen again. Because the x-axis points to the left (but we want to move the image to the right), we have to use a negative offset for the translation:

F3

If we flip the image again, the following happens. The origin is preserved and the x-axis is mirrored:

F4

So we must translate the image by a negative offset:

F5

Another way of undoing the flip is undoing the operations (but in the opposite order):

glTranslatef(width, 0, 0);
glScalef(-1,1,1);

The mathematical reason for that is that inversion reverses the oder. If we have Matrix A = B * C then A^-1 = (C^-1 * B^-1).

Upvotes: 2

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