Question

I’m trying to get the bytes from a RGBA texture in openGL ES. I think I’m trying to imitate glGetTexImage from vanilla openGL. As is right now, the pixels are all nil, from the call to glClear, not the texture data I’m looking for.

This is a method in a category extending SPTexture from Sparrow Framework.

-(NSMutableData *) getPixelsFromTexture
{
    GLsizei width = (GLsizei) self.width;
    GLsizei height = (GLsizei) self.height;
    GLint oldBind = 0;
    glGetIntegerv(GL_TEXTURE_BINDING_2D, &oldBind);
    glBindTexture(GL_TEXTURE_2D, self.textureID);

    GLuint fbo[1];
    glGenFramebuffersOES(1, fbo);
    glBindFramebufferOES(GL_FRAMEBUFFER_OES, fbo[0]);

    GLuint rbo[1];
    glGenRenderbuffersOES(1, rbo);
    glBindRenderbufferOES(GL_RENDERBUFFER_OES, rbo[0]);
    glRenderbufferStorageOES(GL_RENDERBUFFER_OES, GL_RGBA4_OES, width, height);

    glFramebufferRenderbufferOES(GL_FRAMEBUFFER_OES, GL_COLOR_ATTACHMENT0_OES, GL_RENDERBUFFER_OES, rbo[0]);

    GLenum status = glCheckFramebufferStatusOES(GL_FRAMEBUFFER_OES);
    if (status != GL_FRAMEBUFFER_COMPLETE_OES) {
        NSLog(@"Incomplete FBO");
    }

    // draw
    static float texCoords[8] = { 0.0f, 0.0f, 1.0f, 0.0f, 1.0f, 1.0f, 0.0f, 1.0f };
    static float vertexCoords[8] = { -1.0f, -1.0f, 1-.0f, 1.0f, 1.0f, 1.0f, 1.0f, -1.0f };

    glMatrixMode (GL_PROJECTION);
    glPushMatrix();
    glLoadIdentity ();
    glOrthof(0.0f, self.width, self.height, 0.0f, 0.0f, 1.0f);
    glMatrixMode (GL_MODELVIEW);
    glPushMatrix();
    glLoadIdentity();
    glClearColor(0.0f, 0.0f, 0.0f, 0.0f);
    glClear(GL_COLOR_BUFFER_BIT);

    glEnableClientState(GL_TEXTURE_COORD_ARRAY);
    glEnableClientState(GL_VERTEX_ARRAY);

    glTexCoordPointer(2, GL_FLOAT, 0, texCoords);
    glVertexPointer(2, GL_FLOAT, 0, vertexCoords);

    glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);

    glDisableClientState(GL_TEXTURE_COORD_ARRAY);
    glDisableClientState(GL_VERTEX_ARRAY);

    glPopMatrix();
    glMatrixMode(GL_PROJECTION);
    glPopMatrix();
    // end draw

    GLsizei count = width * height * 4;
    GLubyte * pixels = (GLubyte *)malloc((sizeof(GLubyte) * count));
    glReadPixels(0, 0, width, height, GL_RGBA, GL_UNSIGNED_BYTE, pixels);

    glBindRenderbufferOES(GL_RENDERBUFFER_OES, 0);
    glBindFramebufferOES(GL_FRAMEBUFFER_OES, 0);
    glBindTexture(GL_TEXTURE_2D, oldBind);
    glDeleteRenderbuffersOES(1, rbo);
    glDeleteFramebuffersOES(1, fbo);

    return [NSMutableData dataWithBytes:pixels length:count];
}

What am I doing wrong?

Answer 1

So, I was eventually able to solve this using a variation of the code seen here. My code is:

@interface SPRenderTexture (RenderToImage)

- (NSMutableData *)renderToPixelData;

@end

@implementation SPRenderTexture (RenderToImage)

- (NSMutableData *)renderToPixelData
{
    __block NSMutableData * pixels = nil;

    [self bundleDrawCalls:^() 
    {
    float scale = [SPStage contentScaleFactor];
    int width = scale * self.width;
    int height = scale * self.height;
    int nrOfColorComponents = 4; //RGBA
    int rawImageDataLength = width * height * nrOfColorComponents;
    GLenum pixelDataFormat = GL_RGBA;
    GLenum pixelDataType = GL_UNSIGNED_BYTE;

    GLubyte *rawImageDataBuffer = (GLubyte *) malloc(rawImageDataLength);
    glReadPixels(0, 0, width, height, pixelDataFormat, pixelDataType, rawImageDataBuffer);

    pixels = [[NSMutableData alloc] initWithBytes:rawImageDataBuffer length:rawImageDataLength];

    }];

    return [pixels autorelease];
}

@end

I hope this will be useful to other people.