Question

i like to render a simple texture with my fragment shader to 4 vertices and read the image resolution with glReadPixels. I set the (readPixel) size like the (source)picture size but i didn’t get a complete image back. There is always a black bar on the right side. And the image seems to be compressed.

The returned part of the image is correct. It shows the resolution of my sobel shader. So i didn’t think that there are some errors on the ReadPixel part or the SetImage part. But i don’t know…

Here is my method to set the image source:

-(void)setImageSource : (unsigned char*) image
{
static const GLfloat textureVertices[] = 
{
    0.0f, 1.0f,
    0.0f, 0.0f,
    1.0f, 1.0f,
    1.0f, 0.0f,
};

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

glGenTextures(1, &pictureTexture);
glActiveTexture(GL_TEXTURE_2D);
glBindTexture(GL_TEXTURE_2D, pictureTexture);

glUniform1i(uniforms[UNIFORM_VIDEOFRAME], 0);

glVertexAttribPointer(ATTRIB_VERTEX, 2, GL_FLOAT, 0, 0, squareVertices);
glEnableVertexAttribArray(ATTRIB_VERTEX);
glVertexAttribPointer(ATTRIB_TEXTUREPOSITON, 2, GL_FLOAT, 0, 0, textureVertices);
glEnableVertexAttribArray(ATTRIB_TEXTUREPOSITON);

glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);

glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, imageHeight, imageWidth, 0, GL_RGBA, GL_UNSIGNED_BYTE, image);
}

Here the part to render the texture:

-(void)render
{
    glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);
    glViewport(0, 0, imageHeight, imageWidth);

    [self presentFramebuffer];
}

And here is the part to read the resolution back:

-(void)readPixels : (unsigned char*) dest
{            
    glReadPixels(0, 0, imageHeight, imageWidth, GL_RGBA, GL_UNSIGNED_BYTE, dest);

    glDeleteTextures(1, &pictureTexture);
}

I don’t have any idea where i make the error. I’ve searched on this forum and on the khronos group forum but i didn’t get a solution for this (and i didn’t find a case with the same error description).

Maybe another important or confusing information – I’ve also tried to put the code to a c++ class. But when i go outside the Objective C class with the EAGLContext i got the correct picture size back but the resolution is wrong the resolution image contains just snow but without the black bar on the side.

Did someone knew a solution for this error?

Regards,
krikit

Answer 1

i have solved the problem. The error was in the initialization of the OpenGL ES view. The part where i create the rectangle for the initWithFrame.

CGRectMake(0.0f, 0.0f, applicationFrame.size.width, applicationFrame.size.height)

There is the wrong size when i set the size by myself i get a complete picture in the destination.

The other part with the snow on my rendered picture comes from the wrong datatype. When i cast an variable to GLuint it’s not the same then initialize a new GLuint variable and set it with the value… but i don’t know why.

unsigned int j = 20
GLuint i = 20;

glReadPixels(0, 0, j, (GLuint)j, GL_RGBA, GL_UNSIGNED_BYTE, pictureDest);
When i work with the GLuint variables instead of the casted elements everything works fine.

krikit