Save QImage from BYTE buffer segfaults ?

[hskoglund]

Hi, you could try explicitly specify the number of bytes per line:
Edit: also, it could be that your deleting the buffer before the QImage is deleted:

BYTE *buf = new BYTE(imWidth * imHeight);
//Populate buf with data using API call here (not shown)
{
    QImage img(buf, imWidth, imHeight, imWidth, QImage::Format_Grayscale8);
    img.save("image.bmp", "BMP");
}
delete [] buf;

[R-P-H]

@hskoglund said in Save QImage from BYTE buffer segfaults ?:

Hi, you could try explicitly specify the number of bytes per line:
Edit: also, it could be that your deleting the buffer before the QImage is deleted:

BYTE *buf = new BYTE(imWidth * imHeight);
//Populate buf with data using API call here (not shown)
{
    QImage img(buf, imWidth, imHeight, imWidth, QImage::Format_Grayscale8);
    img.save("image.bmp", "BMP");
}
delete [] buf;

Hi, thanks for the suggestion. I tried it and result is the same...still crashes on occasion.

I also tried with removing the delete [] buf line and still the same.

[Bonnie]

@R-P-H
If your BYTE means unsigned char, then the expression of new BYTE(imWidth * imHeight) is definitely wrong.
As @mranger90 says, that really means you allocate a buf of only one byte size, whose value is (imWidth * imHeight).
So I won't be surprised that it crashes.

[R-P-H]

@Bonnie said in Save QImage from BYTE buffer segfaults ?:

@R-P-H
If your BYTE means unsigned char, then the expression of new BYTE(imWidth * imHeight) is definitely wrong.
As @mranger90 says, that really means you allocate a buf of only one byte size, whose value is (imWidth * imHeight).
So I won't be surprised that it crashes.

I tried with new BYTE[imWidth * imHeight] as well but the image is always black.

[Bonnie]

@R-P-H
Right, this is another problem.
How can you be sure your populated data is not all black?
I will suggest you do some tests without Qt classes.

BYTE *buf = new BYTE[imWidth * imHeight];
//Populate buf with data using API call here (not shown)
bool AllZero = true;
for(int i = 0; i < imWidth * imHeight; i++) {
    if(buf[i] != 0)
        AllZero = false;
}
qDebug() << "AllZero :" << AllZero;
delete [] buf;

Or you can just print all of the data to see whether it is all zero(black).

[R-P-H]

@Bonnie said in Save QImage from BYTE buffer segfaults ?:

@R-P-H
Right, this is another problem.
How can you be sure your populated data is not all black?
I will suggest you do some tests without Qt classes.

BYTE *buf = new BYTE[imWidth * imHeight];
//Populate buf with data using API call here (not shown)
bool AllZero = true;
for(int i = 0; i < imWidth * imHeight; i++) {
    if(buf[i] != 0)
        AllZero = false;
}
qDebug() << "AllZero :" << AllZero;
delete [] buf;

Or you can just print all of the data to see whether it is all zero(black).

Yes it's all black everytime.

[R-P-H]

I would like to try save the buffer to a
BMP image using only standard C++ just to check that it's not a problem with using QImage. But I haven't been able to get that working yet...

[hskoglund]

QImage is quite reliable, for example if I add just 2 for loops to change the pixels to something not black:

int imWidth = 260;
int imHeight = 300;

typedef unsigned char BYTE;
BYTE *buf = new BYTE[imWidth * imHeight];

for (int y = 0; (y < imHeight); ++y)
    for (int x = 0; (x < imWidth); ++x)
        buf[x + y * imWidth] = x*x + y*y;

QImage img(buf, imWidth, imHeight, QImage::Format_Grayscale8);

img.save("image.bmp", "BMP");
delete [] buf;

I get:

[hskoglund]

Hi again, dug around in my old projects (turn-of-the-century old :-) and I found this function to write grey BMP files the hard way:

void save_bitmap_grey8(char* file_name, int width, int height, BYTE* pixel_data)
{
    #pragma pack(push, 1)
    struct bitmap_file_header
    {
        short bitmap_type;
        int   file_size;
        short reserved1;
        short reserved2;
        int   offset_bits;
    } bfh;
    #pragma pack(pop)

    struct bitmap_image_header
    {
        int   size_header;
        int   width;
        int   height;
        short planes;
        short bit_count;
        int   compression;
        int   image_size;
        int   ppm_x;
        int   ppm_y;
        int   clr_used;
        int   clr_important;
    } bih;

    struct grey_palette { BYTE r; BYTE g; BYTE b; BYTE a; } gp[256];
    for (int i = 0; (i < 256); ++i)
    {
         gp[i].r = gp[i].g = gp[i].b = i;
         gp[i].a = 0;
    }

    int imagesize = height * (((width * 8 + 31) & ~31) / 8);

    memcpy(&bfh.bitmap_type, "BM", 2);
    bfh.file_size   = sizeof(bfh) + sizeof(bih) + sizeof(gp) + imagesize;
    bfh.reserved1   = 0;
    bfh.reserved2   = 0;
    bfh.offset_bits = sizeof(bfh) + sizeof(bih) + sizeof(gp);

    bih.size_header   = sizeof(bih);
    bih.width         = width;
    bih.height        = height;
    bih.planes        = 1;
    bih.bit_count     = 8;
    bih.compression   = 0;
    bih.image_size    = imagesize;
    bih.ppm_x         = 3780; // meter->inch->96 ppi
    bih.ppm_y         = 3780; // same here
    bih.clr_used      = 256;
    bih.clr_important = 256;

    FILE* image = fopen(file_name, "wb");
    fwrite(&bfh, 1, sizeof(bfh), image);
    fwrite(&bih, 1, sizeof(bih), image);
    fwrite(&gp, 1, sizeof(gp), image);
    fwrite(pixel_data, 1, width * height, image);

    fclose(image);
}

So just swap out QImage with the above function, like this:

int imWidth = 260;
int imHeight = 300;

BYTE *buf = new BYTE[imWidth * imHeight];

for (int y = 0; (y < imHeight); ++y)
    for (int x = 0; (x < imWidth); ++x)
        buf[x + y * imWidth] = x*x + y*y;

save_bitmap_grey8("image.bmp",imWidth,imHeight,buf);

delete [] buf;

Looks exactly the same, except....
it's upside down! That's a relic of OS/2's spec. for bitmaps, they wanted (x =0 , y = 0) to be in lower left corner instead of top left corner. QImage fixes this but my code has that vintage upside/down bug :-)

[R-P-H]

@hskoglund Thanks for the info ! Looks to me like the API is not populating the buffer hence why it is all black.

[R-P-H]

@mranger90 said in Save QImage from BYTE buffer segfaults ?:

try:
BYTE *buf = new BYTE[imWidth * imHeight];

So the issue turned out to be with the device itself. Changing the code as above and replacing the device solved the issue. Thanks.