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gf_alpha_t

Alpha operation specification structure

Synopsis:

See below

Description:

The gf_alpha_t structure describes an alpha operation. This structure is passed as an argument to gf_context_set_alpha and gf_layer_set_blending.

Alpha operations typically involve two multipliers, M1 (applied to the source value) and M2 (applied to the destination value). These multipliers can come from:

For more information, see Layer alpha blending and Alpha and chroma in the Working with Layers, Surfaces, and Contexts chapter.

This structure contains at least the following members:

uint32 mode
The alpha mode to determine the blending/testing operation. See the list of valid flags below.
unsigned map_x_offset, unsigned map_y_offset
The initial horizontal and vertical offset into the alpha map, if an alpha map is used.
gf_surface_t map
The surface containing the alpha map, which is assumed to consist of byte values. Each byte represents an alpha factor ranging from 0 to 255. The surface format must be GF_FORMAT_BYTE.

If neither M1 or M2 come from the alpha map (that is, neither GF_ALPHA_M1_MAP nor GF_ALPHA_M2_MAP is set in mode), this member is ignored.

uint8_t m1
The global M1 factor used when the GF_ALPHA_M1_GLOBAL mode bit is set.
uint8_t m2
The global M2 factor used when the GF_ALPHA_M2_GLOBAL mode bit is set.

Alpha mode flags

These flags set the alpha blending operation. They are:

Alpha Blending multiplier origins:

Flag Description
GF_ALPHA_M1_SRC_PIXEL_ALPHA “M1” comes from the source pixel alpha component, for images only. For draw operations, M1 is 1.
GF_ALPHA_M1_GLOBAL “M1” comes from the global M1
GF_ALPHA_M1_MAP “M1” comes from the Alpha map
GF_ALPHA_M2_DST_PIXEL_ALPHA “M2” comes from the destination pixels alpha component, for images only. For draw operations, M2 is 1.
GF_ALPHA_M2_GLOBAL “M2” comes from the global M2
GF_ALPHA_M2_MAP “M2” comes from the alpha map

Some combinations of multiplier origins are supported. When a combination is specified, the multiplier origin is the product of two origins. For example, GF_ALPHA_M1_SRC_PIXEL_ALPHA | GF_ALPHA_M1_GLOBAL means that “M1” comes from the source pixel alpha component multiplied by the global M1. The supported combinations are:

Alpha Blending Source Factor:

Flag Description OpenGL-ES Equivalent
GF_BLEND_SRC_0 (0,0,0,0) GL_ZERO
GF_BLEND_SRC_M1 (M1,M1,M1,M1) GL_SRC_ALPHA
GF_BLEND_SRC_1mM1 (1-M1,1-M1,1-M1,1-M1) GL_ONE_MINUS_SRC_ALPHA
GF_BLEND_SRC_1mD (1-M2,1-Rd,1-Gd,1-Bd) GL_ONE_MINUS_DST_COLOR
GF_BLEND_SRC_M2 (M2,M2,M2,M2) GL_DST_ALPHA
GF_BLEND_SRC_D (M2,Rd,Gd,Bd) GL_DST_COLOR
GF_BLEND_SRC_1 (1,1,1,1) GL_ONE
GF_BLEND_SRC_1mM2 (1-M2,1-M2,1-M2,1-M2) GL_ONE_MINUS_DST_ALPHA

Alpha Blending Destination Factor:

Flag Description OpenGL-ES Equivalent
GF_BLEND_DST_0 (0,0,0,0) GL_ZERO
GF_BLEND_DST_M1 (M1,M1,M1,M1) GL_SRC_ALPHA
GF_BLEND_DST_1mM1 (1-M1,1-M1,1-M1,1-M1) GL_ONE_MINUS_SRC_ALPHA
GF_BLEND_DST_1mS (1-M1,1-Rs,1-Gs,1-Bs) GL_ONE_MINUS_SRC_COLOR
GF_BLEND_DST_M2 (M2,M2,M2,M2) GL_DST_ALPHA
GF_BLEND_DST_S (M1,Rs,Gs,Bs) GL_SRC_COLOR
GF_BLEND_DST_1 (1,1,1,1) GL_ONE
GF_BLEND_DST_1mM2 (1-M2,1-M2,1-M2,1-M2) GL_ONE_MINUS_DST_ALPHA

Classification:

QNX Graphics Framework

See also:

gf_context_set_alpha(), gf_layer_set_blending()

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