I have an HBITMAP containing alpha channel data. I can successfully render this using the ::AlphaBlend GDI function.
However, when I call the ::GetPixel GDI function, I never get back values with an alpha component. The documentation does say that it returns the RGB value of the pixel.
Is there a way to retrieve the alpha channel values for pixels in an HBITMAP?
I want to be able to detect when to use ::AlphaBlend, and when to use an old-school method for treating a particular colour in the source HBITMAP as transparent.
HDC sourceHdc = ::CreateCompatibleDC(hdcDraw);
::SelectObject(sourceHdc, m_hbmp);
// This pixel has partial transparency, but ::GetPixel returns just RGB.
COLORREF c = ::GetPixel(sourceHdc, 20, 20);
// Draw the bitmap to hdcDraw
BLENDFUNCTION bf1;
bf1.BlendOp = AC_SRC_OVER;
bf1.BlendFlags = 0;
bf1.SourceConstantAlpha = 0xff;
bf1.AlphaFormat = AC_SRC_ALPHA;
::AlphaBlend(di.hdcDraw, x, 10, 64, 64, sourceHdc, 0, 0, 64, 64, bf1);
::DeleteDC(sourceHdc);
Answer
Use GetDIBits to retrieve the first (or more) scan line(s) of the image:
byte* bits[1000];// = new byte[w * 4];
BITMAPINFO bmi;
memset(&bmi, 0, sizeof(BITMAPINFO));
bmi.bmiHeader.biSize = sizeof(BITMAPINFOHEADER);
bmi.bmiHeader.biWidth = w;
bmi.bmiHeader.biHeight = -h;
bmi.bmiHeader.biBitCount = 32;
bmi.bmiHeader.biPlanes = 1;
bmi.bmiHeader.biCompression = BI_RGB;
bmi.bmiHeader.biSizeImage = 0;
bmi.bmiHeader.biXPelsPerMeter = 0;
bmi.bmiHeader.biYPelsPerMeter = 0;
bmi.bmiHeader.biClrUsed = 0;
bmi.bmiHeader.biClrImportant = 0;
int rv = ::GetDIBits(sourceHdc1, m_hbmp, 0, 1, (void**)&bits, &bmi, DIB_RGB_COLORS);
//bits[3] == alpha of topleft pixel;
//delete[] bits;
Use GetDIBits. That way you get an array of RGBQUAD's which have as you can probably guess an alpha channel next to the R, G and B components.
Related
I'm an absolute beginner at this but have managed to blunder my way to 93% of where I want to be. Need help for the final 7%.
I've manually created a bitmap like so:
BITMAPINFO bmpInfo = { 0 };
BITMAPINFOHEADER bmpInfoHeader = { 0 };
BITMAP ImageBitmap;
void *bits;
bmpInfoHeader.biSize = sizeof(BITMAPINFOHEADER);
bmpInfoHeader.biBitCount = 32;
bmpInfoHeader.biClrImportant = 0;
bmpInfoHeader.biClrUsed = 0;
bmpInfoHeader.biCompression = BI_RGB;
bmpInfoHeader.biHeight = -IMAGE_DISPLAY_HEIGHT;
bmpInfoHeader.biWidth = IMAGE_DISPLAY_WIDTH;
bmpInfoHeader.biPlanes = 1;
bmpInfoHeader.biSizeImage = IMAGE_DISPLAY_WIDTH * IMAGE_DISPLAY_HEIGHT * 4;
ZeroMemory(&bmpInfo, sizeof(bmpInfo));
bmpInfo.bmiHeader = bmpInfoHeader;
bmpInfo.bmiColors->rgbBlue = 0;
bmpInfo.bmiColors->rgbGreen = 0;
bmpInfo.bmiColors->rgbRed = 0;
bmpInfo.bmiColors->rgbReserved = 0;
g_hImageBitmap = CreateDIBSection(hDC, &bmpInfo, DIB_RGB_COLORS, &bits, NULL, 0);
GetObject(g_hImageBitmap, sizeof(BITMAP), &ImageBitmap);
for (i = 0; i < IMAGE_DISPLAY_WIDTH; i++) {
for (j = 0; j < IMAGE_DISPLAY_HEIGHT; j++) {
((unsigned char *)bits)[j*IMAGE_DISPLAY_WIDTH * 4 + i * 4] = 255; // Blue
((unsigned char *)bits)[j*IMAGE_DISPLAY_WIDTH * 4 + i * 4 + 1] = 255; // Green
((unsigned char *)bits)[j*IMAGE_DISPLAY_WIDTH * 4 + i * 4 + 2] = 255; // Red
((unsigned char *)bits)[j*IMAGE_DISPLAY_WIDTH * 4 + i * 4 + 3] = 0;
}
}
g_ImageBitmapPixels = bits;
and elsewhere WM_PAINT handles drawing this like so
hdc = BeginPaint(hwnd, &ps);
if (g_hImageBitmap != NULL) {
GetObject(g_hImageBitmap, sizeof(BITMAP), &bm);
hOldBitmap = (HBITMAP)SelectObject(hMemoryDC, g_hImageBitmap);
BitBlt(hdc, UPPER_LEFT_IMAGE_X, UPPER_LEFT_IMAGE_Y,
bm.bmWidth, bm.bmHeight, hMemoryDC, 0, 0, SRCCOPY);
SelectObject(hMemoryDC, hOldBitmap);
}
Given the global variable g_ImageBitmapPixels other parts of the program can change and manipulate individual pixels in the bitmap, and when that happens, I use
InvalidateRect(hwnd, &RECT_ImageUpdate_Window, TRUE);
UpdateWindow(hwnd);
to update just that little portion of the screen. Works great. Hooray for me.
To get to the point, my question is, if a function has ONLY the HBITMAP (g_hImageBitmap) ... is there a way to call the Windows library functions to draw lines, text, circles, filled circles, to the HBITMAP? Like these functions
MoveToEx(hDC, x1, y1, NULL);
LineTo(hDC, x2, y2 );
HBRUSH hRedBrush = CreateSolidBrush(RGB(255, 0, 0));
FillRect(hDC, &somerectangle, hRedBrush);
except instead of needing a device context, they just take the HBITMAP?
I have a pointer to the actual pixels (g_ImageBitmapPixels) so I could just write my own line drawing, circle drawing, rectangle filling functions. Indeed I have done that, but it seems a shame not to use the functions Microsoft so kindly provides. Also, I'm not smart enough to make my own text-drawing functions.
Thank you for your help.
i am doing a program where you take a screenshot of a window and then scan every pixel of that picture. But I have a problem assigning RGBQUAD array to the taken screen. Every pixel has the same RGB which is 205. Here is a piece of my code:
RGBQUAD *pixel = malloc((ssWidth * ssHeight)* sizeof(RGBQUAD));
hdcScreen = GetDC(gameHandle);
hdc = CreateCompatibleDC(hdcScreen);
hBmp = CreateCompatibleBitmap(hdcScreen, ssWidth, ssHeight);
SelectObject(hdc, hBmp);
BitBlt(hdc, 0, 0, ssWidth, ssHeight, hdcScreen, xCenter, yCenter, SRCCOPY);
GetDIBits(hdc, hBmp, 0, ssHeight, pixel, &bmpInfo, DIB_RGB_COLORS);
int p = -1;
for(y_var = 0; y_var < ssWidth; y_var++)
{
for(x_var = 0; x_var < ssHeight; x_var++)
{
if(ComparePixel(&pixel[++p]))
{
SetCursorPos(xCenter + x_var + 3, yCenter + y_var + 3);
}
}
}
bool ComparePixel(RGBQUAD *pixel)
{
printf("%d, %d, %d\n"; pixel -> rgbRed, pixel -> rgbGreen, pixel -> rgbBlue);
return false;
}
ComparePixel(RGBQUAD *pixel) function just checks the RGB values. How do i assign the RGBQUAD to the bitmap of the screenshot?
Multiple issues.
The RGBQUAD **pixel = malloc(... and free(*pixel) appear to be the problem. I think you want RGBQUAD *pixel = malloc((ssWidth * ssHeight)* sizeof(RGBQUAD)); (only 1 *)
Suspect the pixels in GetDIBits() s/b pixel.
I think you want y_var = 0; (x_var = 0; also)
ComparePixel() is not defined, but I think you want something closer to if(ComparePixel(pixel[x_var+(y_var*ssWidth)], the_pixel_to_compare_against))
The free(*pixel); s/b _after the 2 for loops and should be free(pixel);
I have a 2752x2200 bitmap image. I can only display 1/3 of it on my MFC dialog box (for obvious size issues), so if I don't scale the image I only get the top-left 917x733 block (the top-left 1/3 block). I want to zoom the image out by a factor of 3 so that the whole image is dislayed in an area the size of 1/3 of the image. I have set up the grayscale bitmap like so:
//////////////////////////////////////////////////////////////////////////
////////// Setup Bitmap ////////////////////////////////////////////////
//////////////////////////////////////////////////////////////////////////
//// FILEHEADER ////
BITMAPFILEHEADER* bf = new BITMAPFILEHEADER;
bf->bfType = 0x4d42;
bf->bfSize = 6054400 + 54 + sizeof(BITMAPINFO);
bf->bfOffBits = 54;
//// INFOHEADER ////
BITMAPINFOHEADER* bi = new BITMAPINFOHEADER;
bi->biSize = 40;
bi->biWidth = 2752;
bi->biHeight = -2200;
bi->biPlanes = 1;
bi->biBitCount = 8;
bi->biCompression = 0;
//bi->biSizeImage = 6054400; //not required
bi->biXPelsPerMeter = 2835;
bi->biYPelsPerMeter = 2835;
bi->biClrUsed = 0;
bi->biClrImportant = 0;
//// INFO ////
BITMAPINFO* pbmi = (BITMAPINFO*)alloca( sizeof(BITMAPINFOHEADER) +
sizeof(RGBQUAD)*256);
pbmi->bmiHeader.biSize = sizeof (pbmi->bmiHeader);
pbmi->bmiHeader.biWidth = 2752;
pbmi->bmiHeader.biHeight = -2200;
pbmi->bmiHeader.biPlanes = 1;
pbmi->bmiHeader.biBitCount = 8;
pbmi->bmiHeader.biCompression = BI_RGB;
pbmi->bmiHeader.biSizeImage = 0;
pbmi->bmiHeader.biXPelsPerMeter = 14173;
pbmi->bmiHeader.biYPelsPerMeter = 14173;
pbmi->bmiHeader.biClrUsed = 0;
pbmi->bmiHeader.biClrImportant = 0;
//create grayscale color palette
for(int i=0; i<256; i++)
{
pbmi->bmiColors[i].rgbRed = i;
pbmi->bmiColors[i].rgbGreen = i;
pbmi->bmiColors[i].rgbBlue = i;
pbmi->bmiColors[i].rgbReserved = 0;
}
//// IMAGE DATA ////
pFrame->GetImage(m_imageData);
//////////////////////////////////////////////////////////////////////
////// Create image that's printed to dialog box /////////////////////
//////////////////////////////////////////////////////////////////////
HDC hdc = ::GetDC(NULL);
hbit = CreateDIBitmap(hdc, bi, CBM_INIT, m_imageData,
pbmi, DIB_RGB_COLORS);
And then I'm drawing the bitmap onto my dialog box like this:
BITMAP* bi = new BITMAP;
CBitmap bmp;
bmp.Attach(hbit);
CClientDC dc(pWnd);
CDC bmDC;
bmDC.CreateCompatibleDC(&dc);
CBitmap *pOldbmp = bmDC.SelectObject(&bmp);
bmp.GetBitmap(bi);
dc.StretchBlt(384,21,bi->bmWidth,bi->bmHeight,&bmDC,0,0,
bi->bmWidth,bi->bmHeight,SRCCOPY);
bmDC.SelectObject(pOldbmp);
The image looks fine with this code, but it's only the top left block of the full image:
In my attempt to scale the image down, I changed the line:
dc.StretchBlt(384,21,bi->bmWidth,bi->bmHeight,&bmDC,0,0,
bi->bmWidth,bi->bmHeight,SRCCOPY);
to:
dc.StretchBlt(384,21,bi->bmWidth/3,bi->bmHeight/3,&bmDC,0,0,
bi->bmWidth,bi->bmHeight,SRCCOPY); // 1/3 original size
Now my output looks zoomed out and it's showing the whole image(good), but everything looks distorted (bad):
(Note the circular rings around the border of the image. Those shouldn't be there and when you actually see the live video of my image stream, they pulsate and basically ruin the image).
My question is: What is causing this distortion and is there something simple I can do to fix it?
EDIT: After trying StretchDIBits():
StretchDIBits(dc.m_hDC, 384, 21, bi->bmWidth/3, bi->bmHeight/3, 0,
0,bi->bmWidth,bi->bmHeight, myObv->GetImageData(),
myObv->GetPBMI(), DIB_RGB_COLORS, SRCCOPY);
my output looked like this:
i.imgur.com/DA49P8x.png
If you are zooming the bitmap, the windows api may not perform well visually because it is designed to optimized for performance. Add the following line before StretchBlt to enhance the bitmap operation:
SetStretchBltMode(dc.m_hDC, HALFTONE);
Instead of StretchBLT try StretchDIBits:
StretchDIBits(dc.m_hDC, 384, 21, bi->bmWidth/3, bi->bmHeight/3, 0, 0,
bi->bmWidth, bi->bmHeight, m_imageData, pbmi, DIB_RGB_COLORS, SRCCOPY);
When I set up and create a 24-bit bitmap like this:
//fileheader
BITMAPFILEHEADER* bf = new BITMAPFILEHEADER;
bf->bfType = 0x4d42;
bf->bfSize = 6054400 + 54;
bf->bfOffBits = 54;
//infoheader
BITMAPINFOHEADER* bi = new BITMAPINFOHEADER;
bi->biSize = 40;
bi->biWidth = 2752;
bi->biHeight = -733;
bi->biPlanes = 1;
bi->biBitCount = 24;
bi->biCompression = 0;
//bi->biSizeImage = 6054400;
bi->biXPelsPerMeter = 2835;
bi->biYPelsPerMeter = 2835;
bi->biClrUsed = 0;
bi->biClrImportant = 0;
pFrame->GetImage(m_imageData);
//
//create bitmap...
//(hbit is a global variable)
BITMAPINFO* bmi;
bmi = (BITMAPINFO*)bi;
HDC hdc = ::GetDC(NULL);
hbit = CreateDIBitmap(hdc, bi, CBM_INIT, m_imageData, bmi, DIB_RGB_COLORS);
I get an output image like this:
But when I change bitcount from 24 to 8 (which also allows for 3x image size, allowing me to go from 733 width to the image's natural width of 2200), I get an image like this (along with a lot of instability):
My output looks like this:
BITMAP* bi = new BITMAP;
CBitmap bmp;
bmp.Attach(hbit);
CClientDC dc(pWnd);
CDC bmDC;
bmDC.CreateCompatibleDC(&dc);
CBitmap *pOldbmp = bmDC.SelectObject(&bmp);
bmp.GetBitmap(bi);
dc.BitBlt(384,26,bi->bmWidth/3,bi->bmHeight,&bmDC,0,0,SRCCOPY);
//note: if bitcount is 8, height and width need to be /3,
//if 24, only width gets /3
bmDC.SelectObject(pOldbmp);
//explicitly delete everything just to be safe
delete bi;
DeleteObject(bmp);
DeleteObject(dc);
DeleteObject(pOldbmp);
DeleteObject(bmDC);
So my questions are:
Why is this happening when I switch from 24 to 8?
Is there an easy way to output the image as monochrome rather than color?
One last thing:
My coworker wrote this function a long time ago for a similar issue, but he said I may be able to use it. I can't get it to work, unfortunately:
void CopyMono8ToBgrx(byte* pDestBlue, byte* pDestGreen, byte *pDestRed, byte* pDestAlpha)
{
byte* pSrc;
byte* pSrcEnd;
pSrc = ( byte* ) m_imageData;
pSrcEnd = pSrc + ( 2752*2200 );
while ( pSrc < pSrcEnd )
{
byte data = *pSrc;
*pDestBlue = data;
*pDestGreen = data;
*pDestRed = data;
*pDestAlpha = 255; // alpha is always 255 (fully opaque)
pSrc++;
pDestBlue += 4;
pDestGreen += 4;
pDestRed += 4;
pDestAlpha += 4;
}
}
You should create a color pallete. Try this:
struct BITMAPINFO256 {
BITMAPINFOHEADER bmiHeader;
RGBQUAD bmiColors[256];
} bmi;
memset(&bmi, 0, sizeof(BITMAPINFO256));
bmi.bmiHeader.biSize = 40;
bmi.bmiHeader.biWidth = 2752;
bmi.bmiHeader.biHeight = -733;
bmi.bmiHeader.biPlanes = 1;
bmi.bmiHeader.biBitCount = 8;
bmi.bmiHeader.biCompression = 0;
bmi.bmiHeader.biXPelsPerMeter = 2835;
bmi.bmiHeader.biYPelsPerMeter = 2835;
bmi.bmiHeader.biClrUsed = 256;
bmi.bmiHeader.biClrImportant = 0;
for (unsigned int i = 0; i < 256; i++) {
bmi.bmiColors[i].rgbRed = i;
bmi.bmiColors[i].rgbGreen = i;
bmi.bmiColors[i].rgbBlue = i;
}
And then when you call CreateDIBitmap it will become:
hbit = CreateDIBitmap(hdc, &bmi.bmiHeader, CBM_INIT, m_imageData, (BITMAPINFO*)&bmi, DIB_RGB_COLORS);
Also note that you should be careful to also increase the offset in your BITMAPFILEHEADER so that it it expresses that there is color pallete defined before the actual pixels data (yesterday I was having hard time because of this, see Creating 8bpp bitmap with GDI and saving it as a file):
bf->bfOffBits = 54 + sizeof(RGBQUAD)*256;
And to that function that your coworker wrote: It's better to use Luminance to convert colors to gray-scale equivalents:
Hope this helps :)
8 bit per pixel images are assuming a color palette following BITMAPINFOHEADER structure (see BITMAPINFO::bmiColors). If you make the palette to be 256 gray shades, the image is going to me 8 bpp grayscale. Now it's color with random colors on it.
The function CopyMono8ToBgrx you quoted creates full color bitmap, with gray individual pixels (R=G=B).
I want to read a rectangular area, or whole screen pixels. As if screenshot button was pressed.
How i do this?
Edit: Working code:
void CaptureScreen(char *filename)
{
int nScreenWidth = GetSystemMetrics(SM_CXSCREEN);
int nScreenHeight = GetSystemMetrics(SM_CYSCREEN);
HWND hDesktopWnd = GetDesktopWindow();
HDC hDesktopDC = GetDC(hDesktopWnd);
HDC hCaptureDC = CreateCompatibleDC(hDesktopDC);
HBITMAP hCaptureBitmap = CreateCompatibleBitmap(hDesktopDC, nScreenWidth, nScreenHeight);
SelectObject(hCaptureDC, hCaptureBitmap);
BitBlt(hCaptureDC, 0, 0, nScreenWidth, nScreenHeight, hDesktopDC, 0,0, SRCCOPY|CAPTUREBLT);
BITMAPINFO bmi = {0};
bmi.bmiHeader.biSize = sizeof(bmi.bmiHeader);
bmi.bmiHeader.biWidth = nScreenWidth;
bmi.bmiHeader.biHeight = nScreenHeight;
bmi.bmiHeader.biPlanes = 1;
bmi.bmiHeader.biBitCount = 32;
bmi.bmiHeader.biCompression = BI_RGB;
RGBQUAD *pPixels = new RGBQUAD[nScreenWidth * nScreenHeight];
GetDIBits(
hCaptureDC,
hCaptureBitmap,
0,
nScreenHeight,
pPixels,
&bmi,
DIB_RGB_COLORS
);
// write:
int p;
int x, y;
FILE *fp = fopen(filename, "wb");
for(y = 0; y < nScreenHeight; y++){
for(x = 0; x < nScreenWidth; x++){
p = (nScreenHeight-y-1)*nScreenWidth+x; // upside down
unsigned char r = pPixels[p].rgbRed;
unsigned char g = pPixels[p].rgbGreen;
unsigned char b = pPixels[p].rgbBlue;
fwrite(fp, &r, 1);
fwrite(fp, &g, 1);
fwrite(fp, &b, 1);
}
}
fclose(fp);
delete [] pPixels;
ReleaseDC(hDesktopWnd, hDesktopDC);
DeleteDC(hCaptureDC);
DeleteObject(hCaptureBitmap);
}
Starting with your code and omitting error checking ...
// Create a BITMAPINFO specifying the format you want the pixels in.
// To keep this simple, we'll use 32-bits per pixel (the high byte isn't
// used).
BITMAPINFO bmi = {0};
bmi.bmiHeader.biSize = sizeof(bmi.bmiHeader);
bmi.bmiHeader.biWidth = nScreenWidth;
bmi.bmiHeader.biHeight = nScreenHeight;
bmi.bmiHeader.biPlanes = 1;
bmi.bmiHeader.biBitCount = 32;
bmi.bmiHeader.biCompression = BI_RGB;
// Allocate a buffer to receive the pixel data.
RGBQUAD *pPixels = new RGBQUAD[nScreenWidth * nScreenHeight];
// Call GetDIBits to copy the bits from the device dependent bitmap
// into the buffer allocated above, using the pixel format you
// chose in the BITMAPINFO.
::GetDIBits(hCaptureDC,
hCaptureBitmap,
0, // starting scanline
nScreenHeight, // scanlines to copy
pPixels, // buffer for your copy of the pixels
&bmi, // format you want the data in
DIB_RGB_COLORS); // actual pixels, not palette references
// You can now access the raw pixel data in pPixels. Note that they are
// stored from the bottom scanline to the top, so pPixels[0] is the lower
// left pixel, pPixels[1] is the next pixel to the right,
// pPixels[nScreenWidth] is the first pixel on the second row from the
// bottom, etc.
// Don't forget to free the pixel buffer.
delete [] pPixels;
Rereading your question, it sounds like we may have gotten off on a tangent with the screen capture. If you just want to check some pixels on the screen, you can use GetPixel.
HDC hdcScreen = ::GetDC(NULL);
COLORREF pixel = ::GetPixel(hdcScreen, x, y);
ReleaseDC(NULL, hdcScreen);
if (pixel != CLR_INVALID) {
int red = GetRValue(pixel);
int green = GetGValue(pixel);
int blue = GetBValue(pixel);
...
} else {
// Error, x and y were outside the clipping region.
}
If you're going to read a lot of pixels, then you're better off with a screen capture and then using GetDIBits. Calling GetPixel zillions of times will be slow.
You make a screenshot with BitBlt(). The size of the shot is set with the nWidth and nHeight arguments. The upper left corner is set with the nXSrc and nYSrc arguments.
You can use the code below to read the screen pixels:
HWND desktop = GetDesktopWindow();
HDC desktopHdc = GetDC(desktop);
COLORREF color = GetPixel(desktopHdc, x, y);
HBITMAP is not a pointer or an array, it is a handle that is managed by Windows and has meaning only to Windows. You must ask Windows to copy the pixels somewhere for use.
To get an individual pixel value, you can use GetPixel without even needing a bitmap. This will be slow if you need to access many pixels.
To copy a bitmap to memory you can access, use the GetDIBits function.