CODEWITHSUNDEEP
iphoneiosopengl-escoordinatestextures
Sven
Sven

Reputation: 153

iOS: Confusion about texture coordinates for rendering a texture on the screen

I am trying to render a texture that was generated by the camera on the iPhone screen. I downloaded the color tracking example from Brad Larson on http://www.sunsetlakesoftware.com/2010/10/22/gpu-accelerated-video-processing-mac-and-ios (direct link for sample code: http://www.sunsetlakesoftware.com/sites/default/files/ColorTracking.zip).

In the ColorTrackingViewController drawFrame method he uses the following code to generate vertex and corresponding texture coordinates for rendering a textured square:

static const GLfloat squareVertices[] = {
    -1.0f, -1.0f,
    1.0f, -1.0f,
    -1.0f,  1.0f,
    1.0f,  1.0f,
};

static const GLfloat textureVertices[] = {
    1.0f, 1.0f,
    1.0f, 0.0f,
    0.0f,  1.0f,
    0.0f,  0.0f,
};

I don't understand why these texture coordinates work correctly.

In my opinion, and in another example code I have seen that works also correctly, they should be:

static const GLfloat textureVertices[] = {
    0.0f, 1.0f,
    1.0f, 1.0f,
    0.0f,  0.0f,
    1.0f,  0.0f,
};

I went through the whole code, but I cannot figure out why the above texture coordinates work correctly. What am I missing?

Upvotes: 0

Views: 279

Answers (1)

mattg8000
mattg8000

Reputation: 26

I believe it is because the image data from the iphone camera is always presented rotated 90 CCW. To counteract that rotation he's setting the texture co-ordinates to be rotated 90 CCW too. Sometimes two wrongs do make a right?

Upvotes: 1

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