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Andrea · Guest

Hi, all.

I' try-ing to read some pixel color in a visible control.

I try it with a canvas, but it return always -1.

canv-Pixels[0][0];

I see the x,y coordinate are on ClipRect ( 0,0 - 243,149) so I don't
understand why the code return -1.

Any hints?

Many thanks,

Andrea Peri.

Damon Chandler (TeamB) · Guest

Hi Andrea,
When you call the Pixels property on a TCanvas object for a particular
window, you need to make sure that the point in the window is visible
and not covered by another window or a child windowed control. For
example, calling Form1->Canvas->Pixels[x][y] will return -1 if some
other window or windowed control (e.g., a TPanel) occupies the point
(x,y) on Form1. You can get around this limitation by using a Canvas
associated with the screen...

#include <memory>
void __fastcall TForm1::Button1Click(TObject *Sender)
{
std::auto_ptr<TCanvas> ScreenCanvas(new TCanvas());
ScreenCanvas->Handle = GetDC(NULL);
try
{
TPoint const P = ClientOrigin;
ShowMessage(ScreenCanvas->Pixels[P.x][P.y]);
}
__finally
{
ReleaseDC(NULL, ScreenCanvas->Handle);
ScreenCanvas->Handle = NULL;
}
}

HTH,
Damon (TeamB)

Andrea wrote: