Alphakeyword

Parameter to the Put graphics statement which selects alpha blending as the method

Syntax

Put [ target, ] [ STEP ] ( x,y ), source [ ,( x1,y1 )-( x2,y2 ) ], Alpha

Put [ target, ] [ STEP ] ( x,y ), source [ ,( x1,y1 )-( x2,y2 ) ], Alpha, alphaval

Parameters

Name		Description
`alphaval`		Optional alpha parameter in the range [0..255]. Overrides alpha values in individual pixels.

Description

Alpha selects alpha blending as the method for Putting an image. If the alphaval parameter is specified, it overrides the alpha value of each pixel, and the mask color (magenta) will be treated as transparent. This works in 15, 16, 24, or 32-bit color depths.

If alphaval is not specified, Alpha will only work in 32-bit color depth, and Put will use the alpha value embedded within each pixel. Pixels using the mask color will be treated as normal, and drawn with their given alpha value.

Alpha also has another mode which allows an 8-bit image to be Put on top of a 32-bit image. In this case, it will replace the alpha channel of the 32-bit image with the contents of the 8-bit image.

Alpha values range between 0 and 255. An alpha value of 0 will not draw the image at all. All other alpha values are incremented by 1 to get a range between 2 and 256, and the result is then divided by 256 to get a value between 1/128 and 1, which is used to calculate the exact value of each pixel from the source and destination pixels. Thus, 255 is practically equivalent to drawing using Put with Trans blitting mode, 0 is equivalent to doing nothing at all, and all the other alpha values blend is expected.

Remarks

Differences from QB

New to FreeBASIC

Example

This example compares the two different Alpha modes, including how they react to the mask color

'' Set up a 32-bit screen

ScreenRes 320, 200, 32



'' Draw checkered background

For y As Integer = 0 To 199

    For x As Integer = 0 To 319

        PSet (x, y), IIf((x Shr 2 Xor y Shr 2) And 1, RGB(160, 160, 160), RGB(128, 128, 128))

    Next x

Next y



'' Make image sprite for Putting

Dim img As Any Ptr = ImageCreate(32, 32, RGBA(0, 0, 0, 0))

For y As Single = -15.5 To 15.5

    For x As Single = -15.5 To 15.5

        Dim As Integer r, g, b, a

        If y <= 0 Then

            If x <= 0 Then

                r = 255: g = 0: b = 0   '' red

            Else

                r = 0: g = 0: b = 255   '' blue

            End If

        Else

            If x <= 0 Then

                r = 0: g = 255: b = 0   '' green

            Else

                r = 255: g = 0: b = 255 '' magenta (transparent mask color)

            End If

        End If

        a = 255 - (x ^ 2 + y ^ 2)

        If a < 0 Then a = 0': r = 255: g = 0: b = 255

        PSet img, (15.5 + x, 15.5 - y), RGBA(r, g, b, a)

    Next x

Next y



'' Put with single Alpha value, Trans for comparison

Draw String (32, 10), "Single alpha"

Put (80 - 16,  50 - 16), img, Alpha, 64

Put (80 - 16, 100 - 16), img, Alpha, 192

Put (80 - 16, 150 - 16), img, Trans



'' Put with full Alpha channel

Draw String (200, 10), "Full alpha"

Put (240 - 16, 100 - 16), img, Alpha



'' Free the image memory

ImageDestroy img



'' Wait for a keypress

Sleep

This example shows the special method for setting a 32-bit alpha channel using an 8-bit image

Dim As Any Ptr img8, img32

Dim As Integer x, y, i



'' Set up an 8-bit graphics screen

ScreenRes 320, 200, 8

For i = 0 To 255

    Palette i,  i, i, i

Next i

Color 255, 0



'' Create an 8-bit image

img8 = ImageCreate(64, 64, 0,  8)

For y = 0 To 63

    For x = 0 To 63

        Dim As Single x2 = x - 31.5, y2 = y - 31.5

        Dim As Single t = Sqr(x2 ^ 2 + y2 ^ 2) / 5

        PSet img8, (x, y), Sin(t) ^ 2 * 255

    Next x

Next y



Draw String (16, 4), "8-bit Alpha sprite"

Put (16, 16), img8

Sleep





'' Set up a 32-bit graphics screen

ScreenRes 320, 200, 32

For y = 0 To 199

    For x = 0 To 319

        PSet (x, y), IIf(x - y And 3, RGB(160, 160, 160), RGB(128, 128, 128))

    Next x

Next y



'' Create a 32-bit, fully opaque sprite

img32 = ImageCreate(64, 64, 0, 32)

For y = 0 To 63

    For x = 0 To 63

        PSet img32, (x, y), RGB(x * 4, y * 4, 128)

    Next x

Next y



Draw String (16, 4), "Original Alpha channel"

Put (16, 16), img32, Alpha



'' Put a new alpha channel using the 8-bit image

Put img32, (0, 0), img8, Alpha



Draw String (16, 104), "New Alpha channel"

Put (16, 116), img32, Alpha



''Free the memory for the two images

ImageDestroy img8

ImageDestroy img32



Sleep

Reference

Documented in KeyPgAlphaGfx.html