my situation is that i receive different images from the server onto the iPhone client.
what i need to do is to add a Fisheye effect to those images.
what my hallucination is that i can build a dome object, then attach images to it as they come to me.
if anyone can give me any pointers to the right direction i'll be thankful.
Have you tried looking at Paul Bourke's example code for OpenGL? I don't know if it's compatible with OpenGL-ES. Note the feedback about using "glCopyTexSubImage2D instead of doing a slow glReadPixels".
AFAICT, Paul Bourke is doing essentially what you said, creating a distorted grid (your "dome object") to lay the image on. According to this question, that's a good approach.
Updated:
Or, since iPhone's OpenGL-ES supports shaders (at least in some versions), you could get better performance using a plane distortion technique like this. You'd just have to change the formulas for uv.x and uv.y. Then you wouldn't need to be concerned about breaking up the image into a grid of small polygons... you'd have single-pixel resolution for free. :-)
Hm... maybe you need to define what kind of "fisheye" effect you are looking for. Seems there is more than one.
See also this question.
Updated again:
Here is some GLSL code I wrote to do a lens effect in a shader. I believe it's a hemispherical fisheye lens. It runs under WebGL, which uses OpenGL ES, so it should be adaptable for iPhone.
#ifdef GL_ES
precision highp float;
#endif
uniform vec2 resolution;
uniform vec4 mouse;
uniform sampler2D tex0;
// lens
void main(void)
{
vec2 p = gl_FragCoord.xy / resolution.xy;
vec2 m = mouse.xy / resolution.xy;
float lensSize = 0.4;
vec2 d = p - m;
float r = sqrt(dot(d, d)); // distance of pixel from mouse
vec2 uv;
if (r >= lensSize) {
uv = p;
} else {
// Thanks to Paul Bourke for these formulas; see
// http://paulbourke.net/miscellaneous/lenscorrection/
// and .../lenscorrection/lens.c
// Choose one formula to uncomment:
// SQUAREXY:
// uv = m + vec2(d.x * abs(d.x), d.y * abs(d.y));
// SQUARER:
uv = m + d * r; // a.k.a. m + normalize(d) * r * r
// SINER:
// uv = m + normalize(d) * sin(r * 3.14159 * 0.5);
// ASINR:
// uv = m + normalize(d) * asin(r) / (3.14159 * 0.5);
}
vec3 col = texture2D(tex0, vec2(uv.x, -uv.y)).xyz;
gl_FragColor = vec4(col, 1.0);
}
To test this, paste it into ShaderToy in a WebGL-enabled browser. Set input 0 to a texture like http://www.iquilezles.org/apps/shadertoy/presets/tex0.jpg or http://www.iquilezles.org/apps/shadertoy/presets/tex4.jpg
Click the play button, and drag the mouse around on the rendering area. The lens is centered on the mouse pointer.
