How to read screen to see which pixels are white - iphone

Question

I have a view on which the user can draw some lines, which have been developed using this.
Now the lines are drawn between points using the code:
- (void) renderLineFromPoint:(CGPoint)start toPoint:(CGPoint)end
{
static GLfloat* vertexBuffer = NULL;
static NSUInteger vertexMax = 64;
NSUInteger vertexCount = 0,
count,
i;
//Allocate vertex array buffer
if(vertexBuffer == NULL)
vertexBuffer = malloc(vertexMax * 2 * sizeof(GLfloat));
// Add points to the buffer so there are drawing points every X pixels
count = MAX(ceilf(sqrtf((end.x - start.x) * (end.x - start.x) + (end.y - start.y) * (end.y - start.y)) / kBrushPixelStep), 1);
for(i = 0; i < count; ++i) {
if(vertexCount == vertexMax) {
vertexMax = 2 * vertexMax;
vertexBuffer = realloc(vertexBuffer, vertexMax * 2 * sizeof(GLfloat));
}
vertexBuffer[2 * vertexCount + 0] = start.x + (end.x - start.x) * ((GLfloat)i / (GLfloat)count);
vertexBuffer[2 * vertexCount + 1] = start.y + (end.y - start.y) * ((GLfloat)i / (GLfloat)count);
vertexCount += 1;
}
//Render the vertex array
glVertexPointer(2, GL_FLOAT, 0, vertexBuffer);
glDrawArrays(GL_POINTS, 0, vertexCount);
// Display the buffer
[self swapBuffers];
}
The objective is to read the drawing area of the screen which is initiated by the following code:
PictureView * scratchPaperView = [[RecordedPaintingView alloc] initWithFrame:CGRectMake(0, 45, 320, 415)];
[self.view addSubview:scratchPaperView];
I want to find out the pixels of the lines, i.e. all the pixels that are white in the drawing area. Please tell me how to proceed from here?

Answer 1

Assuming that you can get a UIImage.CGImage or a CGImageRef out of a PictureView, then you render this image into a CGBitMapContext. The image is going to tell you the number of components and if it has alpha and where alpa is. Most likely you are going to get 4 byte pixels (32 bits/pixel). You then walk each row looking at each pixel. Assuming a black background (which would be 255,0,0,0 or 0,0,0,255), you will see non-black pixels when you get close to or hit a line. A pure with pixel is going to be 255,255,255,255.
I'm pretty sure you can find examples of how to render an image into a context, and also how to examine pixels by googling around. Frankly what always gets me is the confusing pixel layout attributes - I usually end up printing a few test cases out to make sure I got it right.