Technical Information Database
TI797D.txt Bitmaps And InterBase BLOB Fields
Category :Database Programming
Platform :All
Product :Delphi All
Description:
dBASE and Paradox tables provide a BLOB field to store binary data that,
when the stored data is of bitmap-format, work as-is with the TDBImage
component to display images. In Database Desktop, these field types are
listed as Binary and Graphic (for dBASE and Paradox tables, respectively).
However, the process of storing bitmap images in InterBase BLOB fields
and using the stored data with TDBImage components is not as straight-
forward.
InterBase tables do not have just one type of BLOB field. There are three
variants, or sub-types: type 0, type 1, and user-defined sub-types. Types
0 and 1 are pre-defined types. Type 0 BLOB fields (the default type) are
for storing general binary data. Type 1 BLOB fields are for storing text
BLOB data. Either the pre-defined type 0 or a user-defined BLOB sub-type
will allow the automated retrieval of bitmap data from the BLOB field that
is to be displayed in a TDBImage component. Type 0 BLOB fields may be used
for storing bitmap-format data or raw binary data. Here is an example of
manually extracting bitmap data stored in a type 0 BLOB field (Table1-
BLOBField) and displaying the data in a (non-data-aware) TImage component:
procedure TForm1.ExtractBtnClick(Sender: TObject);
begin
Image1.Picture.Bitmap.Assign(Table1BLOBField);
end;
This manual method may be used or, more commonly, a data-aware control
would be used so that the display of a given record's bitmap (in a BLOB
field) will be automatically displayed. The TDBImage serves this purpose,
and by setting the DataSource property to the TDataSource component
associated with the underlying table and setting the DataField to the BLOB
field containing the bitmap, the TDBImage component will display the image
stored in the BLOB field and automatically load each record's BLOB field
contents as the record pointer is changed.
The Database Desktop utility will allow the creation only of type 0 binary
BLOB fields, no provision was made for user-defined BLOB field sub-types.
If it is desired that a user-defined BLOB sub-type be used to store the
bitmap data, it would need to be created with an SQL statement. Typically
this would be through the WISQL utility, but an appropriate SQL statement
in a TQuery would suffice. Here is an SQL statement that creates a table
with a user-defined BLOB field sub-type:
CREATE TABLE WITHBMP
(
FILENAME CHAR(12),
BITMAP BLOB SUB_TYPE -1
)
Once a table with a compatible BLOB field is created, storing bitmap data
to the BLOB field and displaying the bitmap images in a TDBImage component
uses the same methods as would be used with dBASE or Paradox tables.
There are a number of ways to load a bitmap image into a BLOB field. Three
of the easier methods involve 1) copying the data from the Windows clip-
board into a TDBImage component connected to the BLOB field, 2) using the
LoadFromFile method of the TBLOBField component, and 3) using the Assign
method to copy an object of type TBitmap into the Picture property of a
TBDBImage.
The first method, copying the bitmap from the clipboard, is probably most
handy when an application needs to add bitmaps to a table when the end-
user is running the application. A TDBImage component is used to act as an
interface between the BLOB field in the table and the image stored in the
clipboard. The PasteFromClipboard method of the TDBImage component is
invoked to copy the bitmap data from the clipboard into the TDBImage. When
the record is posted, the image is stored into the BLOB field in the
table.
Because the Windows clipboard can contain data in formats other than just
bitmap, it is advisable to check the format prior to calling the CopyFrom-
Clipboard method. To do this, a TClipboard object is created and its Has-
Format method is used to determine if the data in the clipboard is indeed
of bitmap format. Note that to use a TClipboard object, the Clipbrd unit
must be included in the Uses section of the unit that will be creating
the object.
Here is an example showing the contents of the clipboard being copied into
a TDBImage component, if the contents of the clipboard are of bitmap
format:
procedure TForm1.Button1Click(Sender: TObject);
var
C: TClipboard;
begin
C := TClipboard.Create;
try
if Clipboard.HasFormat(CF_BITMAP) then
DBImage1.PasteFromClipboard
else
ShowMessage('Clipboard does not contain a bitmap!');
finally
C.Free;
end;
end;
The second method of filling a BLOB field with a bitmap involves loading
the bitmap directly from a file on disk into the BLOB field. This method
lends itself equally well to uses at run-time for the end-user as for
the developer building an application's data. This method uses the Load-
FromFile method of the TBLOBField component, the Delphi representation of
an InterBase BLOB field.
The LoadFromFile method of the TBLOBField component requires a single
parameter: the name of the bitmap file to load, which is of type String.
The value for this parameter may come from a number of sources from the
end-user manually keying in a valid file name to the program providing a
string to the contents of the FileName property of the TOpenDialog comp-
onent.
Here is an example showing the use of the LoadFromFile method for a TBLOB-
Field component named Table1Bitmap (a field called Bitmap in the table
associated with a TTable component named Table1):
procedure TForm1.Button2Click(Sender: TObject);
begin
Table1Bitmap.LoadFromFile(
'c:\delphi\images\splash\16color\construc.bmp');
end;
The third method uses the Assign method to copy the contents of an object
of type TBitmap into the Picture property of a TDBImage component. An
object of type TBitmap might be the Bitmap property of the Picture object
property of a TImage component or it may be a stand-alone TBitmap object.
As with the method copying the data from the clipboard into a TDBImage
component, the bitmap data in the TDBImage component is saved into the
BLOB field in the table when the record is successfully posted.
Here is an example using the Assign method. In this case, a stand-alone
TBitmap object is used as the source of the bitmap data. To put a bitmap
image into the TBitmap, the LoadFromFile method of the TBitmap component
is called.
procedure TForm1.Button3Click(Sender: TObject);
var
B: TBitmap;
begin
B := TBitmap.Create;
try
B.LoadFromFile('c:\delphi\images\splash\16color\athena.bmp');
DBImage1.Picture.Assign(B);
finally
B.Free;
end;
end;
Going the opposite direction -- extracting a bitmap from an InterBase BLOB
field (without first saving the bitmap out to a file) is a simple process
of using the Assign method of the TBLOBField object to store the contents
of the BLOB field to an object of type TBitmap. A stand-alone TBitmap
object or the Bitmap property of the Picture object property of a TIMage
component are examples of compatible destinations for this operation.
Here is an example demonstrating using the Assign method to copy a bitmap
from a BLOB field into a TImage component (Table1Bitmap is the TBLOBfield
for the BLOB field in the table).
procedure TForm1.Button1Click(Sender: TObject);
begin
Image1.Picture.Bitmap.Assign(Table1Bitmap);
end;
In this example, the TBLOBField object Table1Bitmap is a BLOB field in an
InterBase table. This TBLOBField object was created using the Fields
Editor. If the Fields Editor is not used to create TFields for the fields
in the table, the fields must be referenced using either the FieldByName
method or the Fields property, both part of the TTable and TQuery comp-
onents. In cases where one of those means is used to reference the BLOB
field in a table, the field reference must be type-cast as a TBLOBField
object prior to using the Assign method. For example:
procedure TForm1.Button1Click(Sender: TObject);
begin
Image1.Picture.Bitmap.Assign(TBLOBField(Table1.Fields[1]));
end;
A bitmap stored in a BLOB field may also be copied directly to a stand-
alone TBitmap object. Here is an example showing the creation of a
TBitmap object and storing into it a bitmap from a BLOB field.
procedure TForm1.Button2Click(Sender: TObject);
var
B: TBitmap;
begin
B := TBitmap.Create;
try
B.Assign(Table1Bitmap);
Image1.Picture.Bitmap.Assign(B);
finally
B.Free;
end;
en
Reference:
7/16/98 4:33:54 PM
Last Modified: 01-SEP-99