C / C++ / MFC

Re: What`s a palette in the parsing a bitmap context

10-Mar-20 7:05

Exactly, this allows to save space by 1) only referencing used colours in the palette 2) using colour references instead of raw values.
This feature is used in .gif files, as well as in icons, IIRC. It is not used in .bmp files, which store colour information uncompressed. It may be used in other formats as well.

"Five fruits and vegetables a day? What a joke!
Personally, after the third watermelon, I'm full."

harold aptroot10-Mar-20 16:31

harold aptroot

10-Mar-20 16:31

Unfortunately (I would call it that, because it's a mess) BMP files are in theory quite general, with tons of annoying features being piled onto it. Using any of those features results in a BMP file that a lot of software can't read (or reads differently than intended) because almost everyone treats BMP as only 24bit-uncompressed with no fancy extras. But anyway, in theory, a BMP file can indicate that it has 8 or fewer bits per pixel and then it must have a color table aka palette. The pixel data can even be compressed then.

Re: What`s a palette in the parsing a bitmap context

phil.o10-Mar-20 19:27

Re: What`s a palette in the parsing a bitmap context

10-Mar-20 19:27

Thanks for the update. I never had to work with bmps other than plain 24 bpp (or 8 bpp for b/w) uncompressed data.

"Five fruits and vegetables a day? What a joke!
Personally, after the third watermelon, I'm full."

Calin Negru10-Mar-20 21:06

Calin Negru

10-Mar-20 21:06

thanks for your help. So biBitCount less than 9 it`s no longer what you would consider a bitmap

Re: What`s a palette in the parsing a bitmap context

phil.o10-Mar-20 21:11

Re: What`s a palette in the parsing a bitmap context

10-Mar-20 21:11

Yes I would still consider it as a bitmap.
I was specifically talking about .bmp files. The general notion of bitmap is something different.

"Five fruits and vegetables a day? What a joke!
Personally, after the third watermelon, I'm full."

Calin Negru10-Mar-20 21:30

Calin Negru

10-Mar-20 21:30

my bad, I meant it`s no longer a .bmp

Re: What`s a palette in the parsing a bitmap context

phil.o10-Mar-20 21:46

Re: What`s a palette in the parsing a bitmap context

10-Mar-20 21:46

But apparently I'd be wrong, as harold aptroot told, .bmp format is way more generic than that.

"Five fruits and vegetables a day? What a joke!
Personally, after the third watermelon, I'm full."

Calin Negru11-Mar-20 7:32

Calin Negru

11-Mar-20 7:32

I understand

Re: What`s a palette in the parsing a bitmap context

leon de boer12-Mar-20 5:15

leon de boer

12-Mar-20 5:15

It is rare but even 24bit and 32bit bitmaps can have palettes, you do it if you want something like millions of shades of blue for example when printing an ocean scene.

If you look at the structure of the BitmapFileInfo header .. look at the second last entry that is how you know how many palette colours there are ... the last entry tells you how many are important.

bitmapinfoheader[^]

It is spelled out very clearly ...

If biClrUsed is zero, the array contains the maximum number of colors for the given bitdepth

You assume a 24bit or 32bit bitmap have no palette at your own risk.

In vino veritas

Having issues with the cin.getline() function in C++

Member 147670008-Mar-20 14:29

Re: Having issues with the cin.getline() function in C++

8-Mar-20 14:29

for a short test in my class, we had to write code about pigs that allowed the user to enter information about a set of pigs (e.g. the pigs' names, breed, weight, gender, etc.). We were to create a class called pigs that would be able to hold the pigs' information. There was nothing wrong with the class itself, the problem I'm having is actually in the main function.

cout << "Enter the pig gender (\'m\'/\'f\')(if not press \'enter\' for none): ";
cin.getline(pigGender, 10);
if(pigGender != "\n")
{
    while(pigGender != "m" && pigGender != "f")
    {
        cout << "Invalid gender, please enter a valid gender m or f(if not press \'Enter\'): ";
        cin.getline(pigGender, 10);
        if(pigGender != "\n")
        {
            //cout << "if branch, pigGender:" << pigGender;
            gender = pigGender[0];
        }
        else
        {
            //cout << "else branch, pigGender:" << pigGender;
            gender = ' ';
            break;
        }
    }
    pigs[i].SetPigGender(gender);
}

The piece of code above, shows how I'm trying to obtain the pig's gender from the user. I made it so that if the user doesn't know the gender, they can just hit enter to move on (as I did with other features that worked fine). The problem is that when you hit enter, the pigGender string doesn't seem to take the newline character. Also, I understand how the getline function works (takes in a newline and stops), hence I believe when you hit enter, the getline will take in the newline and stop, but here it does not seem to take it in at all. I also made it so that if the user enters a wrong letter (not m or f), then they can still hit enter if they do not know. The commented out codes are checks I implemented to see exactly what was in pigGender. When I enter a charcter that is not a newline it will go to the if branch and dislpay my error message, however, when I hit only enter, it still goes through that if branch instead of the else branch. the checks i commented out show me which branch I am in and the charcter in the pigGender string, and when I hit enter for the newline, there seems to be nothing in the pigGender string. What seems even more bizarre, is that the following code is very similar to what I did above (I actually copied and pasted it and made some changes to make the one above) and it works perfectly.

cout << "Enter the pig weight(if not press \'Enter\'): ";
cin.getline(weight, 10);
if(weight != "\n")
{
    float pigWeight = atof(weight);
    while(pigWeight < 0)
    {
        cout << endl;
        cout << "Invalid weight, please enter a valid weight(if not press \'Enter\'): ";
        cin.getline(weight, 10);
        if(weight != "\n")
        {
            pigWeight = atof(weight);
        }
        else
        {
            pigWeight = 0;
        }
    }
    pigs[i].SetPigWeight(pigWeight);
}

Is there something I'm just not seeing here, because I have been looking at this code since yesterday and have tried a bunch of stuff to fix it but can't. Of course, I do know of many other ways to tackle this problem (like comparing string size instead), but I started with this and its really bugging me that it does not work, so I just want to figure it out just so I can know. If you can help me understand, please respond, I really want to figure this thing out, thank you!

k50548-Mar-20 15:37

Re: Having issues with the cin.getline() function in C++

8-Mar-20 15:37

iostream::getline() strips the trailing delimiter from input. That means that when you hit return only, the string is empty (e.g = "").

You also have an error in all your string comparisons:

C++

if(weight != "\n")

is always true (this actually compares the address of weight to the address of the static object "\n". To compare objects of type char * (or char[]), you must use strcmp(). In this case, since you are just interested to see if the only char in the string is '\n' you could say

C++

if(weight[0] != '\n')

(note that this still won't work because iostream::getline() strips the newline delimiter from the input.

It only looks like the weight is being skipped. As explained above the comparison is always true, so the code block following the comparison is always executed. What is happening is that

C++

pigweight = atof(weight)

evaluates with weight = "", which evaluates to zero, so it only seems like the code block was skipped. If you were to change the code to, say

C++

if(weight != "\n") // This is always true ...
{
    cerr << "checking weight from user\n";
    // rest of block 
}

You would always see the "checking weight from user" message, whether you hit return or not.

Member 147670009-Mar-20 10:55

Re: Having issues with the cin.getline() function in C++

9-Mar-20 10:55

But when I enter a valid weight and hit enter, the program moves on to the next feature (it doesnt go through the if branch).

Member 147670009-Mar-20 11:01

Re: Having issues with the cin.getline() function in C++

9-Mar-20 11:01

I think I get what you are saying, but I just ran the program again and if I enter a valid weight (positive number) everything runs normally and at the end it displays the number I entered.

k50549-Mar-20 12:12

Re: Having issues with the cin.getline() function in C++

9-Mar-20 12:12

Sure, but if you do this

C++

if(weight != "\n")
{
  // ... 
  pigs[i].SetPigWeight(pigWeight);
}
else
    pigs[i].SetPigWeight(100.0); // Set default weight to 100.0
// ...

Now, when you hit enter, you would expect that the default weight would be used (i.e. set to 100.0), if the comparison is working as you expect. But it won't be, it will be set to zero. Believe me when I tell you that

C++

if (weight != "\n")

does not compare objects of type char* (ie c-style strings). You must use strcmp() to compare c-style strings. Better than using iostream::getline, take a look at std::getline. Your code might then look something like

C++

#include <string>
// ...
string input_string;
// ...
getline(cin, input_string);
if(input_string.length() != 0) // User entered a value
{
    // ...
}

Furthermore, what happens if you enter one hundred for the weight? It doesn't get set to 100, but zero, since atof("one hundred") == 0.0. A good solution should check that the input is a number. At the very least that the input string contains only the characters "0123456789.". Ideally you would check that there was only one decimal point, too.

Member 1476700010-Mar-20 8:49

Re: Having issues with the cin.getline() function in C++

10-Mar-20 8:49

You're right, I copied and pasted that code to try it and it set the pig weight to the default of 0 instead of 100. I understand what you are saying now. Thank you so much for taking out the time to explain this to me in detail. This thing was really frustrating me and you really cleared it up for me, I really appreciate this!

k505410-Mar-20 9:04

Re: Having issues with the cin.getline() function in C++

10-Mar-20 9:04

You're very welcome.

String comparisons can be tricky in C/C++. I have over 30 years experience with C and it took me a couple of readings of your code to realize

C++

if(weight != "\n")

did not do what it looks like it should! I'm sure I'm not the only one who looked at that and didn't twig right away that the string comparisons were incorrect.

Stefan_Lang8-Mar-20 22:26

Stefan_Lang

8-Mar-20 22:26

k5054 already said it all.

But I'd like to add the general advice that you read up on the description of all I/O functions rather than making assumptions about what they return. Especially input functions.

Also, it is usually easier to read std::string objects rather than reading input of unknown length into limited size buffers. You can use the function getline(istream&, string) for that. See getline (string) - C++ Reference[^]

GOTOs are a bit like wire coat hangers: they tend to breed in the darkness, such that where there once were few, eventually there are many, and the program's architecture collapses beneath them. (Fran Poretto)

Re: Having issues with the cin.getline() function in C++

k50549-Mar-20 12:29