Paint app, using framebuffer to render texture in OpenGL ES

Question

I'm trying to make simple paint application, based on Apple's GLPaint. To draw a line, GLPaint draws an array of points with brush texture. With blending in OpenGL turned on, every point of that line with alpha less than 1 is blended with previous ones. I want to avoid this. For example, you set red color to brush and alpha to 0.5, draw a line and all the line is single-color - red with alpha 0.5, without self crossings... I can't explain clear, but I hope you've understood me))

So, first question is how can I do this?

I've decided to draw current line to texture, without blending, and then put over current image. The code is:

- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event {
        CGRect  bounds = [mainView bounds];
         UITouch* touch = [[event touchesForView:mainView] anyObject];
        firstTouch = YES;       location = [touch locationInView:mainView];
        location.y = bounds.size.height - location.y;

         // Offscreen buffer
    glGenRenderbuffersOES(1, &brushFramebuffer);
        glBindFramebuffer(GL_FRAMEBUFFER_OES, brushFramebuffer);
        glGenRenderbuffers(1, &brushDepthBuffer);
        glBindRenderbuffer(GL_RENDERBUFFER, brushDepthBuffer);
        glRenderbufferStorage(GL_RENDERBUFFER, GL_DEPTH_COMPONENT16_OES, backingWidth, backingHeight);
        glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_DEPTH_ATTACHMENT, GL_RENDERBUFFER, rushDepthBuffer);

        GLenum status = glCheckFramebufferStatus(GL_FRAMEBUFFER);
        if(status != GL_FRAMEBUFFER_COMPLETE) {
                NSLog(@"failed to make complete framebuffer object %x", status);
                return;
        }
        glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
}

- (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event {
        CGRect  bounds = [mainView bounds];
        UITouch* touch = [[event touchesForView:mainView] anyObject];
        if (firstTouch) {
                firstTouch = NO;
                previousLocation = [touch locationInView:mainView];
                previousLocation.y = bounds.size.height - previousLocation.y;
        }
         else {
                location = [touch locationInView:mainView];
                location.y = bounds.size.height - location.y;
                previousLocation = [touch previousLocationInView:mainView];
                previousLocation.y = bounds.size.height - previousLocation.y;
        }

        [self renderLineFromPoint:previousLocation toPoint:location];
}

 - (void)touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event {
         CGRect bounds = [mainView bounds];
         UITouch* touch = [[event touchesForView:mainView] anyObject];
        if (firstTouch) {
                firstTouch = NO;
                previousLocation = [touch previousLocationInView:mainView];
                previousLocation.y = bounds.size.height - previousLocation.y;
                [self renderLineFromPoint:previousLocation toPoint:location];
         }

          // getting texture from buffer
        glBindTexture(GL_TEXTURE_2D, bufferTexture);
        glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
        glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, 1024, 768, 0, GL_RGBA, GL_UNSIGNED_BYTE, NULL);
        glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, bufferTexture, 0);

        glBindFramebufferOES(GL_FRAMEBUFFER_OES, viewFramebuffer);
        glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);

        glFramebufferRenderbufferOES(GL_FRAMEBUFFER_OES, GL_COLOR_ATTACHMENT0_OES, GL_RENDERBUFFER_OES, viewRenderbuffer);

        // drawing texture to image
        CGFloat vertices[] = {backingWidth/2, backingHeight/2};

        glVertexPointer(2, GL_FLOAT, 0, vertices);
        glDrawArrays(GL_POINTS, 0, 1);

        glBindRenderbufferOES(GL_RENDERBUFFER_OES, viewRenderbuffer);

        [context presentRenderbuffer:GL_RENDERBUFFER_OES];
}
- (void) renderLineFromPoint:(CGPoint)start toPoint:(CGPoint)end {
        [EAGLContext setCurrentContext:context];
        glBindFramebufferOES(GL_FRAMEBUFFER_OES, brushFramebuffer);
        glBindTexture(GL_TEXTURE_2D, brushTexture);
          /// .... then drawing a line from "start" to "end"
}

The second question is: this code draws only draws brush texture to the center of screen, what am I doing wrong?

Answer 1

there is a simple way of how to do this with lines (with no brush): simply use depth test (clean depth buffer, set depth func to GL_NOT_EQUAL or keep it at GL_LESS) to avoid rendering on the same position twice. However, that would not work with more complex brushes.

Your code has several principal flaws. First, if you want to render to a texture using FBO, you need to call glFramebufferTexture2D() before you do any rendering to the buffer (because that functions tells OpenGL that it should direct rendering output to that texture from that point on, it doesn't do any copying). So the order of operations in touchesBegan become:

glGenRenderbuffersOES(1, &brushFramebuffer);
glBindFramebuffer(GL_FRAMEBUFFER_OES, brushFramebuffer);
// creates FBO

glGenRenderbuffers(1, &brushDepthBuffer);
glBindRenderbuffer(GL_RENDERBUFFER, brushDepthBuffer);
glRenderbufferStorage(GL_RENDERBUFFER, GL_DEPTH_COMPONENT16_OES, backingWidth, backingHeight);
glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_DEPTH_ATTACHMENT, GL_RENDERBUFFER, rushDepthBuffer);
// creates and attaches RB for depth

glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, bufferTexture, 0);
// attaches texture to render to. now we're ready.

Next, you don't call glTexImage2D() in touchesEnded, that would erase all the rendering done to that texture so far. It is okay to call glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR); but you would be better off to move that line to your init section where you create bufferTexture. To stop rendering to framebuffer, one simply calls:

glBindFramebufferOES(GL_FRAMEBUFFER_OES, 0);

The texture contains the image rendered while FBO was active, and drawing returned to the screen (the application framebuffer).

Now, because you were drawing using brushTexture, you need to:

glBindTexture(GL_TEXTURE_2D, bufferTexture);

And then you just render fullscreen quad:

CGFloat vertices[] = {0, 0,
                      backingWidth, 0,
                      backingWidth, backingHeight,
                      0, backingHeight};
CGFloat texCoords[] = {0, 0,
                      1, 0,
                      1, 1,
                      0, 1,};
glVertexPointer(2, GL_FLOAT, 0, vertices);
glTexCoordPointer(2, GL_FLOAT, 0, texCoords);
glDrawArrays(GL_QUADS, 0, 4);

And that's it ...

In renderLineFromPoint() it is not required to call

glBindFramebufferOES(GL_FRAMEBUFFER_OES, brushFramebuffer);

as it was already bound in touchesBegan. But it won't hurt.

And one more thing - you should make sure that the touchesBegan, touchesMoved and touchesEnded really come in that order. I'm not sure this is guaranteed.