using pixel coordinates in opengl es 2

[guich]

Hi,

I downloaded and played with the opengl sample at
http://developer.android.com/training/graphics/opengl/draw.html

However, it uses the standard coordinate system for opengl, where 0,0
is at the center of the screen and -1,-1 at top-left and 1,1 at bottom-
right.

I want to use the screen's coordinate, where 0,0 is at top-left and
width,height is at bottom-right.

I already tried a dozen things, read about 30 articles, but since i'm
a newbie at opengl, i'm still stumped.

Can someone tell what it has to be done to change the coordinate so
that the given rectangle

static float squareCoords[] = { 10f, 10f, 0.0f, // top left
10f, 20f, 0.0f, // bottom left
20f, 20f, 0.0f, // bottom right
20f, 10f, 0.0f }; // top right

... is shown at screen?

thanks

guich

[Harri Smått]

Hi,

I might be wrong but I think you can create an orthographic projection matrix for this purpose using Matrix.orthoM. Just provide it with top=0, bottom=height, left=0, right=width.

--
H

Post by guich
Hi,
I downloaded and played with the opengl sample at
http://developer.android.com/training/graphics/opengl/draw.html
However, it uses the standard coordinate system for opengl, where 0,0
is at the center of the screen and -1,-1 at top-left and 1,1 at bottom-
right.
I want to use the screen's coordinate, where 0,0 is at top-left and
width,height is at bottom-right.
I already tried a dozen things, read about 30 articles, but since i'm
a newbie at opengl, i'm still stumped.
Can someone tell what it has to be done to change the coordinate so
that the given rectangle
static float squareCoords[] = { 10f, 10f, 0.0f, // top left
10f, 20f, 0.0f, // bottom left
20f, 20f, 0.0f, // bottom right
20f, 10f, 0.0f }; // top right
... is shown at screen?
thanks
guich
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[Latimerius]

You could do that and set the matrix via glLoadMatrix(), however a
more usual way would be to call

glOrthof (0, w, h, 0, ...);

Check glOrhtof() docs and note I'm passing height for bottom and zero
for top which achieves the swap.

(Also note that GLES can't render quads directly. Your square has to
be rendered as two triangles.)

Post by Harri Smått
Hi,
I might be wrong but I think you can create an orthographic projection matrix for this purpose using Matrix.orthoM. Just provide it with top=0, bottom=height, left=0, right=width.
--
H

Post by guich
Hi,
I downloaded and played with the opengl sample at
http://developer.android.com/training/graphics/opengl/draw.html
However, it uses the standard coordinate system for opengl, where 0,0
is at the center of the screen and -1,-1 at top-left and 1,1 at bottom-
right.
I want to use the screen's coordinate, where 0,0 is at top-left and
width,height is at bottom-right.
I already tried a dozen things, read about 30 articles, but since i'm
a newbie at opengl, i'm still stumped.
Can someone tell what it has to be done to change the coordinate so
that the given rectangle
static float squareCoords[] = { 10f, 10f, 0.0f, // top left
10f, 20f, 0.0f, // bottom left
20f, 20f, 0.0f, // bottom right
20f, 10f, 0.0f }; // top right
... is shown at screen?
thanks
guich
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